Grating noise from g'box/ prop shaft

1987 520i E28. I've got an intermittent kind of "grating" noise from gearbox/propshaft area which seems to kick in after a few minutes driving.
It is most noticeable at speeds between 15-30mph, and seems to get less obvious at higher speeds, although maybe it's just being masked by the higher background noise. It feels like it's a dry or damaged bearing, and I can feel it transmitted through the gear stick handle.
I am wondering it is is one of: - Gearbox rear bearing - Flexible coupling between g'box and Prop shaft - Front U-Joint bearings - Centre prop shaft bearing
None of these looks particularly attractive to tackle, especially since I would be working from underneath with the car up on axle stands., and I would prefer not to drop the exhaust. Can anyone suggest a way of homing in on the problem? Thanks Steve

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