Grinding from rear and loss of power on acceleration

My latest 'development'. A harsh hollow grinding sound from the rear somewhere on acceleration. Worse in higher revs and when just starting out. After some driving it eases off quite a lot to the point it is barely noticeable (but is still there). I also lose power - the worse the noise is the more power I lose. Gut feeling is flywheel.

Grateful for any pointers?

Have you jacked up and spun a rear wheel whilst listening for any noise?
Thinking diff maybe.

Just an update. The van has been running fine - the grinding noise was my dodgy gypsy exhaust fix! It vibrates noisily but only at certain revs and uncertain times. I have to sort this out properly now anyway as MOT time is fast approaching.