Whining vibration / grumbling noise

[Widge]

So, I picked up my new hardtop for Z3I yesterday, but thats another story. Incedentally for any Cab owners outh ther pondering a hardtop, just do it, they are great bits of kit.

I am now hearing a new sound in the car ('89 325i Cab 185,00 miles). It is a kind of rumbling vibration which can be heard, and felt, and appears to be coming from the prop / diff area. It only starts at 40mph, and has dissappeared by 80mph, so is road speed related. It seems pretty constant in magnatude throughout that range. If I dip the clutch, or back off on the accelerator it stops immediatly, so is only whilst the drivetrain is under load.

I have recently replaced the driveshaft guilbo (Bilbo? Silly name) so I am wondering if somthing is unbalanced or if I have disturbed the central bearing form its hibernation. Or maybe one of the bolts needs nipping up again to prevent vibes. I did everything up very tight, but you never know.

If it isn't just something needing doing up then my diagnosis is that it could be the propshaft bearing, or a bearing in the differential.

Any one else had this on their car, and if so what was it in the end?

[Widge]

Oh
Thought I'd be a good boy and check the Wiki

Bearing

The centre bearing is essential for holding the prop to the bottom of the car, and is mounted inside a carrier frame. Depending on the model, the bearing fits around either the front or rear half of the propshaft.

When making adjustments to the prop, it is essential to tension the centre bearing; this means applying pressure to it before it is bolted in place, otherwise it will simply fall apart.

If the centre bearing is on the front section of the prop (as in 325i, 318i and), then push the bearing about 3mm towards the gearbox and then tighten the bolts up. If the bearing is on the rear section (as in E30 M3), the bearing is tensioned toward the diff.

I never tensioned the prop bearing, is this probably the source of my problems?

[GDBN]

Sounds like it to me ....
And I agree with the advice on a hardtop ....

G