Mystery alarm-like tone from under the hood

This one has me at a loss, and my husband AND our mechanic are utterly stumped. Mechanic will be getting eyeball deep in the engine later this week (booked solid for a few days) but in the meantime, we have to put up with an incessant high-pitched noise that is audibly akin to a single-tone fire alarm. Just so we're clear on this, it's definitely not the security alarm.

The car's an '08 Nissan Altima 3.5 SE. From what everyone can gather, it's not mechanical in origin. In other words, this isn't some high-pitched whistle from a line leak or a screech from a belt, etc. The sound isn't remotely close.

Hubs and I spent quite a while under the hood before we tried to snag a walk-in at the mechanic (obviously didn't happen today) We think we have it narrowed down to what could possibly be the control unit for the keyless entry.

Maybe.

It's hard to see anything written on where we think the sound is originating from, and Nissan's diagrams are crap. As a side note, if anyone can miraculously find a complete engine compartment diagram showing every single thing under the hood, I'd be extremely grateful.
It's shape and thickness does look fairly dead on for the FOB's control unit. Suffice it to say, if anyone is familiar with the set-up under the hood, it's behind the fuse box on the driver's side, mounted on the passenger-facing side of the casing directly in front of the steering wheel.

There's a few other tidbits that may be helpful. It's always doing this ear-piercing noise, ALWAYS. The battery must be disconnected to cease this god-forsaken racket.
If the brake is lightly depressed, it'll cease. If the brake is fully depressed, it'll also cease. In between, it's still ongoing. We have no idea what this might signify aside from maybe a short somewhere?
Brake fluid is good, brakes themselves are good. Vehicle operation is normal, no new noises, emissions, or sensations, and no indicator dash lights going off. And the FOB's battery was replaced to see if it was maybe some harebrained "Yo, battery in your FOB is dying" alarm, but no luck.

Any ideas? We're at a loss, and can find nothing online describing anything close to this.

Alternator bearing, fan belt carrier pully bearing, leak in the gasket of the EGR valve, Power steering pump or the overload bypass valve on the steering rack, mouse nest in the heater/air conditioner fan, vaccum leak somewhere in the engine compartment. Your fuel vapor system can even do that. water pump can also make that noise. Use a small hose and stick one end to your ear and move it around. You can listen for bearing noise with a screwdriver and put your ear to it but make sure not to get it into the belts.

I have had lots of them over the years that make weird noises. If it only happens when hot it can be the radiator cap bypassing air into the radiator reserve bottle that is empty. Those things usually do not send a trouble code other than the EGR valve.

a reply to: rickymouse

I was going to say alternator too. Had a car that went through 8 in about 4 years. But, that's all I cay say as I am only speaking from my own experience.

Vacuum hose leak maybe.

😕

Ok, the suggestions are helpful, I didn't consider the water pump. However, for whatever it's worth, the battery was disconnected for 8 hours to cease the noise, and that noise immediately started back up as soon as terminal contact was made. Could any of the suggestions you all made still be valid even after sitting disconnected for so long?

Again, I stress that this is akin to a fire alarm, it's that shrill, loud & single-toned.

I really hope it's not the alternator, that s# gets expensive.

originally posted by: Nyiah
Ok, the suggestions are helpful, I didn't consider the water pump. However, for whatever it's worth, the battery was disconnected for 8 hours to cease the noise, and that noise immediately started back up as soon as terminal contact was made. Could any of the suggestions you all made still be valid even after sitting disconnected for so long?

Again, I stress that this is akin to a fire alarm, it's that shrill, loud & single-toned.

I really hope it's not the alternator, that s# gets expensive.

Ok, so the engine is not running, then it is electrical in nature. That is a big difference. Most of the electrical stuff is off, except the horn which is live, try pulling the horn fuse or take out the horn relay, it could be making a small internal connection and putting a little electricity to the horn. Try beeping the horn to see if it triggers the relay to reset.

It could also be a seat belt buzzer stuck partly on, or the headlight buzzer. The seat belt system is only after the key is on, the headlight is not. That also works through a relay too, it could be stuck. Some cars also have a buzzer if you leave the fog lights on when you leave the car. The alarm system can also malfunction, that should set a code though I think.

What kind of car is it? Oh, I didn't see the second half of the OP, I thought it ended at the single word maybe.

The brake thing does not make sense, although there is a sensor on the brake that controls shifting sometimes, but that is usually regulated with the ignition on.

a reply to: Nyiah

Use a piece of wood to listen. Get a board or baseball bat and put it on whatever you think is making the noise and put the other end on your ear. Sounds weird but that's how I diagnose noises from an engine that I can't isolate. Works great.

a reply to: Nyiah

Defective electric steering pump power relay ?

🎑

Try popping relays out to see what relay is involved in the noise. That could help to refine the search a little.

originally posted by: rickymouse

originally posted by: Nyiah
Ok, the suggestions are helpful, I didn't consider the water pump. However, for whatever it's worth, the battery was disconnected for 8 hours to cease the noise, and that noise immediately started back up as soon as terminal contact was made. Could any of the suggestions you all made still be valid even after sitting disconnected for so long?

Again, I stress that this is akin to a fire alarm, it's that shrill, loud & single-toned.

I really hope it's not the alternator, that s# gets expensive.

Ok, so the engine is not running, then it is electrical in nature. That is a big difference.

That's why we're thinking maybe a short somewhere at the least, nothing mechanical really makes any sense for this kind of noise. The "epicenter" of it isn't particularly near any of the suggested engine components.
It totally stops making this fire alarm-like noise when we disconnect the battery.

Most of the electrical stuff is off, except the horn which is live, try pulling the horn fuse or take out the horn relay, it could be making a small internal connection and putting a little electricity to the horn. Try beeping the horn to see if it triggers the relay to reset.

Now THIS could be the issue. We could be blaming the FOB control for something it's not responsible for based on proximity, it's coming from in front of the wheel after all.
Hubs is at work right now, I'll let him know to try that real quick before he clocks in.

It could also be a seat belt buzzer stuck partly on, or the headlight buzzer. The seat belt system is only after the key is on, the headlight is not. That also works through a relay too, it could be stuck. Some cars also have a buzzer if you leave the fog lights on when you leave the car.

Those are also something I wouldn't have thought to consider. I'll suggest he check those, too.

The alarm system can also malfunction, that should set a code though I think.

Yeah, it should send some kind alert to the dash panel, a code or indicator icon, something.

What kind of car is it?

Nissan Altima. Been a great vehicle so far, aside from this grating mystery.

I'm assuming punching in the self-diagnostic code on the starter button/door button would probably be of no help if I can't plug in to the vehicle computer. It was a thought, but I doubt it would be helpful, basically just going about it blind.

Already determined electrical, next step is the fuse box to know where to go next.

It looks like there is a common issue with a trunk switch failure making the alarm go off, not sure if it's related.

The trunk switch that depresses when the door is closed corrodes and fails, sending firing off the alarm, it's pitch may be because it's starting to wear out.

something you guys could check out anyway.


added...

Ok, it looks like there is a tsb on the altima for an annoying whine from the electronic power steering assist pump.
their repair for it was to wrap the motor in insulation, remove the fender and insulate behind it.lol jesus

Take a short hose and put it to your ear and to the motor down by the steering assembly and see.

All that considered, the brake depressed thing says to me it might be the brake booster pump.

Hard to keyboard diagnose this one.

a reply to: Nyiah

Software engineer here -- not a mechanic. In my expert opinion its probably the transmission shift solenoid. Some automatic transmissions shift using pulse width modulation. When you step on the brake it probably cuts out the circuit which is why you dont hear it. If PWM has issues you might also notice the car is shifting hard.

Does it sound like this?

youtu.be...

a reply to: drewlander

haha, dude that noise is obnoxious

originally posted by: drewlander
a reply to: Nyiah

Software engineer here -- not a mechanic. In my expert opinion its probably the transmission shift solenoid. Some automatic transmissions shift using pulse width modulation. When you step on the brake it probably cuts out the circuit which is why you dont hear it. If PWM has issues you might also notice the car is shifting hard.

That also could be, though it's not shifting hard, or having trouble shifting in any manner.

Does it sound like this?

youtu.be...

Heh, I listened to that video this morning. No, it doesn't sound like that, that's a buzzing sound & it's too low.

It sounds more like this, except it's a higher pitch and is continuous, not beeping:

Here's another one to check.

link

the box is located on the drivers side firewall, the thread in that link is pretty good.
good luck

a reply to: Nyiah

That sounds like it may be the headlight switch left on signal. Does it make this sound when the keys are in the ignition?

a reply to: Nyiah

Sounds like the type of audible alarm emitted from a GPS tracking device they commonly use at "buy here pay here" stealerships and you triggered the tamper alarm somehow. Usually they are tied in under the dash, but I dont see why they couldn't be under the hood.

Maybe the previous owner installed one aftermarket. Maybe it was a service fleet vehicle. Hard to say, but I am not aware of any oem alarm under the hood in any car that makes that sound.

originally posted by: Mandroid7


Here's another one to check.

link

the box is located on the drivers side firewall, the thread in that link is pretty good.
good luck

Interesting. Sounds right in text. Have a video link to hear it?

Not a mechanic, but former electronic test engineer here. With all cars so computer controlled these days, some of the issues overlap with computer diagnosis. It could be a dead CMOS battery int he on-board computer. In a computer, when that begins to go, some have a piezoelectric component that makes high pitched whine (its a similar circuit to that in the smoke detector to tell you when the battery is low).

Just an idea.