I havea 96 Nissan SentraGxe and I have a grinding sound on the car it not a constant noise sometimes it does it twice a week and it doesn't do it for a long time it only does it when the car is not someone told me that it could be my wheel bearing are going I would like for someone to explain to me what sounds or noise does the car do when the wheel bearing are going bad if there is a mechanic out there please anwser my question. Cause I told it to a certified mechanic and he doesn't fine any part of the car broken or any part of the car worn out. Help!

jdy