Noisy & vibrating propshaft

[CFS318]

The vehicle is an M10 engined 1987 318i and am having what seems to be some propshaft issues, these being:
a) A hum similar to bearing hum at around 120kmph. The bearings have been checked and are not the culprits.
b) Vibration at low speed around 40kmph.
I think the propshaft on my vehicle is a swivel joint type.
The guibo and centre bearing are 2 years old and have done a maximum of around 15000km so I don't think these to be the offending items.
Can anyone shed some light on what may be causing the above issues and a possible solution.

Many thanks.

[CFS318]

Surely with so many knowledgeable people on this forum someone must have some advice.
I unfortunately don't have the funds to do what BMW done when the car was new that being replacing the prop after various attempts to rectify the problem.

Thanks

[Brianmoooore]

that being replacing the prop after various attempts to rectify the problem.

If this car had the problem from new, and BMW couldn't solve it, then it suggests that there's something inherently wrong with the gearbox or diff. that's causing the issue.
There's a projecting spigot on the front of the prop. that locates in a rubber bush bearing in the gearbox output shaft. This is difficult to see, inside the guibo, but if it has failed, it can give rise to vibrations.
The centre bearing is't usually a problem - it's the rubber support for it that fails, and this will happen prematurely if the prop. is out of balance (incorrect assembly?), or it hasn't been preloaded when the centre bearing was bolted up.

[CFS318]

Many thanks for the advice Brian. I'll check it out - hopefully nothing too serious.