Hi, recently i got my oil and tran. oil change at the shop. soon after that i began to hear some odd sound coming from the bottom of my engine. it gets lounder when i put it into gear or when i turn on the AC. does any1 what that sound could be coming from? it sounds like a piece of metal is vibrating, however i couldnt see anything loose or wrong when i lift up the car. thx for any kind of input.