car sounds like train over tracks when driving??

any idea what this could be? i did post this a while back and someone said it could be engine mount, but i doubt it as they look fine.
i thought it could be the wheel baring, but further searches says the sound get louder when turning, mine just stay constant. it does NOT make a noise (or i cant hear it), while i am accelerating, but when i take my foot off the accelerator (while moving) it starts making the noise, it sounds like a train going over train tacks.
to me it sounds like it is...

car sounds like train over tracks when driving??