It's a 1995 3.5 SR. The re-built engine was put in about 7K miles ago. This happened about 3K miles ago...same situation. I started noticing a humming/vibration at certain rpm's, it starts to become more and more noticeable the longer you drive it. It also has to be warmed-up before the vibration is really noticeable. So this time, it was the same thing.