Can you be a little more specific on what the problem is. Is it when you accelerate or idle, both?
Sounds like what me 'Cuda had when I got it, possible transmission problem?
My car was all good but when under throttle the engine sounded like it was going faster than the car was actually going. I found out that my transmission was slipping very badly, the bands were stuffed. All fixed now.
Hope you get to the bottom of the problem.
Don't sell it, it doesn't sound like a major problem, just a little time needed to pin point the fault.