Man... I don't know. I DO think, however, that you have an opportunity get closer to your motor by looking at it as though "something" isn't where it's supposed to be. ie., spark plug wire, etc. Sounds like your car is trying to suck air threw a straw with a hole on the side. Sucking harder and not achieving enough for it's effort. I'm sure someone here knows where you should start, but I'd try on my own first. Maybe start with connections that you can reach without getting under the car first.

****Edit**** Maybe a motor mount?