Rastus

I'm pretty good at id'ing noises but this one's a little weird. Firstly the engine only has 120k miles on it so it really shouldn't have any noises but it does. Tho it sounded to me like the flexplate noise, but I dc'd the flexplate and it's the same so it's got noghitn go do with the bolts... guess it still could be cracked? Odd thing about it is when you first start it up, first say 15 seconds it's completely silent... then it builds up, and if you increase the RPM very much at all, the noise goes away. Really seems like lifter noise but it seems a little deeper to me, and again sounds like it's coming from the rear. Any help appreciated.