[donv]

OK, let's check terms here... what do you mean by oscillation? Is it just a resonance/sound you hear or something you feel through the car seat or steering wheel as well? If sound, what sort of sound (high pitch, low, rumbling, etc.)?

Going only on the basis of what you've said so far, I'm guessing you have some sort of resonance going between a body part and the exhaust, esp. as it is not speed related but in sync with the RPMs (goes away with backing off gas). Does it happen while standing still and revving the engine to that range? If so, that would definitely rule out anything with the running gear, and you could try having someone stand outside the car to listen and get closer to the source.