Alarming seat vibration

[schnaarf]

Can anyone shed any light on this:

i have a vibration through the seat. The rear view mirror is completely still and can't be felt through the gearknob. It's had new engine mounts, gearbox mounts, guibo, csb.

The vibrations starts around 50 and does it whether the in or out of gear and is speed related. There is also a deep rumble but it doesn't sound like a bearing. It changes sound slightly in overrun but has occasionally been really violent in overrun for a few seconds. Hard to pinpoint where the noise is from.

I'm thinking knackered diff, diff mount or cv joint? It really is noticeable. Almost to the point where I don't want to drive it. Before taking it in, anyone have any thoughts or had something similar?

[Speedtouch]

Jack the car up and spin the wheels, looking for any buckles on both inside and outside of the rims, and bulges in the tyres. Feel/hear for any rumble in the wheel bearings while you're at it.

Is it a convertible? If so, ensure you have the damper weight in the corner of the boot.

[Brianmoooore]

Vibration starting at 50 MPH suggests it's related to the rotation of the wheels, rather than the propshaft.

[schnaarf]

Cheers guys I'll check the wheels - maybe swap them rear to front and see what happens.

[schnaarf]

Update: had the wheels balanced. They said one wheel is slightly misshaped and the tyres aren't perfect - said the middle has worn more than the outer. I'll get new tyres.

There must be 2 problems. The rumble from the rear is getting worse. It can be felt at just over 40 and audible at all speeds over 50. Quietest with only very slight throttle, loudest on hard acceleration and overrun so can't be the tyres. The diff did have a whine but it stopped after changing the subframe and trailing arms bushes. There is also a clunk from the rear when taking up drive with tension on the drive train. The garage couldn't find anything wrong underneath, or any noise when running in the air.

This is my daily car so driving me nuts! And worrying that it's getting worse.

[aimlessrock]

that clunk you have mentioned could be propshaft related. The clunk in mine turned out to be worn and split donuts on the prop.

The vibrations could be linked- could the prop been incorrectly alighted when last put together?

[schnaarf]

it was aligned properly yes.

Finally got it on a ramp and had a look myself. When holding one output and turning the opposite wheel, i can feel what seems like a rough, knackered bearing in the diff. Have I found the source?

[Speedtouch]

Quite possibly. It would be even better if you can run the car while in gear with it up on a ramp, so you can detect exactly where the sound/vibration is coming from...

[schnaarf]

Yeah ran the car on the ramp but couldn't hear anything!

[aimlessrock]

A diff rebuilt sounds like it's on the cards- Gareth did mine a couple of years ago and did a great job.

[schnaarf]

Yeah thinking that. What's the rough cost?