1993 Volvo 940 estate 2.0 turbo 4 speed+overdrive manual.

hi, I would appreciate a few opinions. Ive got a intermittent howling/humming/resonating noise coming from underneath my 940 estate and I dont know what it is.
It only occurs when the car is in motion.
Ive tried to isolate as follows, the following actions don't affect it.

dipping the clutch
putting it in another gear/neutral
pulling on the handbrake
using the brakes gradually/suddenly
going round corners

noise is not constant, seems to occur at variety of speeds though car is mostly used at 30mph but is most noticable between 10-30mph.

started a week ago and is getting progressively louder.

Aggravatingly intermittent, just dropped my son off at cubs - it howled all the way there and half way back (2 miles), got my friend to jump in and check it out and it howled for 100yds and then never again while we drove round the block a couple of times, at least he heard it briefly.

any ideas? Im guessing propshaft but I'm used to squeaking bearings not a humming noise . . .