[V8] Noise under load @ 1800 RPM

[Seamus O'Carey]

I've been trying to get to the bottom of this noise issue since I bought the
car.  I thought maybe it was just a loose exhaust hanger, but I just had it
checked out and the entire exhaust system is solid and all the hangers
intact.

We did discover, though, that the noise comes on only under load (whether
accelerating or while standing still with the car in gear) and very suddenly
just before 1800 RPM, and goes away a little after 2k.  Inside the car, it's
a loud grumble that you can feel through the seat/floorboard.  What
confounded us, however, is you can NOT hear the noise from outside the car.
Not even a hint of it, even with the window open.

The guy at the exhaust shop thought maybe it was coming from the center
differential, just behind the transmission (even though he listened to it
with a stethoscope and still couldn't hear anything).  He mentioned
something about how Subaru owners can pull a fuse that will disable the
center differential in their cars, which would help them diagnose something
like this.  Do we have something similar in the V8?

Seamus