Hey everybody need some help with my 97 f150 4.2L. Had the truck about six months and about three months ago started having a low rattling/grinding noise only when at idle and cold and only when in drive or reverse. Seems to get worse when ac or heat is on. As soon as the the truck is above idle rpm noise stops and after driving for around five minutes noise is gone completely. The noise sounds like a shard of metal inside the tranny being spit around but i cant understand why the noise would go away when higher than idle rpm, Any suggestions?? Thanks

 

take the serpentine belt off and check the pulleys.