Sound of a UFO?

Usually I just post anonymously on here, but this was such an odd item that I felt I should at least post it and see what turns up.

I live in the middle of Pennsylvania and as I was leaving work, a friend of mine and I were standing outside talking. Suddenly there was a reasonably loud "BRRRRRRRRPRPRP" (not burp) sound with a distinct doppler effect that went high overhead crossing the entire length of the sky in about .5-1 second's time. we both looked up simultaneously and looked back at each other with a 'wtf was that' look. It was very fast, whatever it was. I've never actually heard an object that fast. But then, after that, it happened several more times, criss-crossing over us. Now in looking up we saw absolutely nothing each time.

This sound was very distinctive. It was unlike anything I've ever heard. It sounded like a motorcycle going on the freeway (we get those because we work right near it) and when they start ripping on it and going over 100 it has a very distinctive sound from a distance of a half mile or so. It was similar to that only MUCH MUCH faster and seemed to stem from far overhead.

To answer any basic questions I might have missed: It did NOT sound like an engine. It did not sound like a jet. It did not have any visible signature. The sound crossed over from different directions. First was about northeast to southeast, then west to east (I have a compass on my keyring), then north to south - those are the only ones I remember. At one point, it sounded like there were several noises overlapping on the last one. We looked around but could find no other sources for the sound. There was a car running nearby, but that was it. Nothing unusual.

Thought this might be interesting, maybe someone else can add something to this.

All I can imagine is some type of pulse detonation. Another thing might be someone's romote controlled plane etc.

Pulse detonation ran through my head too, but damn. If it was (and I live near an airport, so I'm quite familiar with traditional jet sounds, speeds, how fast they cross the sky, etc) I would figure there'd be two things:

1) More time as it crossed over - I can't even conceive of how fast something would be moving for that sound, and then to cross over from different directions within 5-10 seconds?

2) I would also figure a pulse detonation engine would still have some kind of visual fingerprint - something to indicate it was still using some kind of combustion to propel itself.

I should also mention the time was ~8:10 pm, est.