for me, its been my tranny bellhousing cover. father didn't put it back in right when we swapped trannies and it was clicking. took it off, beat it out with a hammer, and keep having to go back and beat it some more lol

 

Thanks for all the replies.

Yes, the "rate of rattle" increases with engine rpm.

Forgot to add, but I have no check engine lights on at the moment.