May be a Motor Mount, something in the drive train.
It's hard to tell. I have an 08 N\A and it make all kind on noise, but I narrow most of it down to being old. Just kinda get under it and look around, find an open back road and test with windows down, If it's coming from the rear it could be a lose piece of something. Just throwing things out.
Also why are you messing with he valve cover?