2007 Nissan sentra grinding noise

The transmission was replaced and
15,000 miles later I start to hear a
grinding noise. I take it back to the
dealership who replaced it they said it
was the engine mount. I replaced the
engine mount and I can still hear the
grinding noise. It started off making
the sound only when I would start to
accelerate now I am hearing it at
higher speeds. Any idea's??? 2
dealerships swear it's not the tranny.