I hear this sound when I accelerate, but not all the time, it seems to stop intermittently. The best way I can describe it is a moaning or a groaning sound. I have no idea where it is coming from and haven't a clue whether it is as simple as something is loose and vibrating or something I should be worried about. I did take it to my mechanic some months ago and asked him what it could be and he could not pinpoint what it was. I was wondering if anyone else has ever had a similar issue. I included a link to a video of the sound.