Not sure if you've sorted this now but I was thinking a wheel bearing maybe if it's coming from the rear and changes sound when you pick up speed. Jack the car up so the rear wheel is off the ground and give it a wobble, it shouldn't move side to side but will if the wheel bearing is loose/knackered.

Edited 10 July, 201213 yr by Rhys Tudor-Davies

Not sure if you've sorted this now but I was thinking a wheel bearing maybe if it's coming from the rear and changes sound when you pick up speed. Jack the car up so the rear wheel is off the ground and give it a wobble, it shouldn't move side to side but will if the wheel bearing is loose/knackered.

Yep seems to be sorted now.

No idea what was causing it.

I removed the wheel, inspected everything, put it back on and it's been fine since.

I can only suspect that it was a stone or something caught somewhere.

Had a similar experience the last time my car was jacked up at a garage.... The tech "forgot" it was on coilovers and when he took it off the ramp the rear springs missed their slots and made a god awful noise. All sorted once I took it back and the penny had dropped.