Worrying vibration

[jimj]

Last weekend, driving up to Scotland, it became noticeable that there was a worrying vibration that was felt slightly at, say, 40mph, but dramatically increased with speed. This is only apparent when feathering the throttle, i.e. when the transmission is not under negative or positive load. When accelerating, even slightly, or slowing down there is no problem. It`s the same in any gear and also with the clutch depressed or even when free-wheeling out of gear. At around 70mph it feels like the car is running over small to medium corrugations in the road. BBBBRRRRRROUGH like that!
We returned today after 1400+ miles on the Scottish Malts (excellent with 3 Elans taking part) and the vibration is no better nor worse.
What can it be?
Jim
Incidentally, the great Jim Jackson was taking part and finished 18th.

[dusty]

Possibly Propshaft out of balance, see other thread (tick, tick, tick noise) about worn Universal joints

[types26/36]

Vibrations can be caused by many things, I had what I thought was a propshaft inbalance and being too lazy to check it out I carried on driving ........BIG MISTAKE!....doing about 150kph and ALL HELL BROKE LOOSE!.......
Check your tyres! most Elans sit around for years and the tyres degrade even though the tread looks OK .....back to my problem .......doing 150KPH and the vibration got worse .........the picture tells the story!.....the tread had seperated from the tyre although it did'nt deflate it nearly caused a major accident, I managed to bring it to a stop but it was touch and go!
The tyres were about ten years old .......time takes it toll......moral of the story ....dont trust old tyres even though they look O.K.

[nebogipfel]

Propshaft U/J commonly causes the symptom you describe.

Also, look carefully at your tyres, modern tyres seem to develop high spots in the tread (go out of round) which will also cause a vibration.

[jimj]

Yes, thanks for that, but tyre problems would vibrate ALL the time not just when the transmission is not under load. I`m thinking propshaft imbalance or u/j.
Jim

[nebogipfel]

Yes I agree, but I have known an out of shape tyre feel like a balance problem, coming in at a certain speed.

Prop balance would also be unlikely to be affected by on/off load.

As I say my money would be on one or both U/J's.

Unlikely but possible could also be the diff mounts allowing the diff to vibrate against the chassis.

[oldelanman]

It might just be loose hardware in the driveline - worth checking diff flange and driveshaft bolts are tight before you start taking things apart.

[jimj]

Problem solved, oldelanman was spot on. The diff. flange to driveshaft bolts had loosened slightly, despite being nyloc nuts. I`ve tightened them up with a little loctite and the driveline is smooth again. Thanks all.
Jim