Hi,
I am a new member and am posting because I have a problem that is driving me nuts!
It is with a V50 1.8, Registered June 2006, done around 48k miles with me driving from new, no accidents or anything else untoward. So its not even 2 years old yet.

A couple of months ago I hit a “pot-hole” very hard. It immediately destroyed the Front N/S tyre by slamming against the rim so hard the sidewall got torn.
I had it replaced, no comments from the tyre dealer that there was any damage to the wheel or anything else (not even sure if this is relevant).

A few weeks ago I noticed a quite severe vibration at around 70 to 80 mph. My immediate thoughts were to get the front wheels balanced. Did that, no difference.

Sent it into a Volvo dealer. They said (despite it just being done) that the Front N/S wheel was badly out of balance and they re balanced it. No difference. I called up the dealer again. This time I asked to demonstrate the problem to one of their Technicians. Did that, only got up to about 60mph and he said he could feel it. Left it with them, they told me they had omitted to balance the O/S front wheel and that needed doing too, but that the front O/S tyre should also be replaced. Again, no difference.

This weekend I went to a tyre dealer he said the O/S Rear was actually more worn that the O/S Front, so he swapped the two wheels over and fitted a new tyre on the Front. I had explained the problem to him. I also had both front wheels re balanced, the bearings checked for any signs of wear and the wheel alignment checked with their laser alignment equipment. Alignment was there or thereabouts and nothing they would be concerned with, and they generally had a look around for anything loose or worn.

This evening was the first opportunity I had to take it on an open road and get up to speed. Again, as soon as I approach 60mph the vibration starts it gets worse between around 65 and 75 mph. After 80mph I am sure it is still there but the frequency is so rapid it is not as noticeable. I had an envelope on the passenger front seat and you could hear it “rattling” on the seat. It is quite strange, its almost feels like a sort of “buffeting” throughout the whole car. You can feel it through the floor-pan and the seat. You can seem to affect the severity by acceleration. Gentle acceleration, change of speed seems to be worse than when the engine is under load and accelerating hard.

Its going back into Volvo in the morning, but I am concerned they are struggling to find the fault.
Any suggestions?

Your replaced N/S front tyre is most likely faulty and becomes "out of round" due to centrifugal forces on the casing with speed, hence the rhythmic oscillations you feel going through the car. Many new tyres can have flaws or weaknesses in the plies as they can hardly all be tested at speed on the road before they are sold! Try gently putting your foot on the brake when you feel the vibration. A wheel or tyre running out of balance or out of "true" should transmit the shimmy to the disc and be felt in the pedal. However an "out of round" tyre with a mild centrifugal bulge most likely will not be felt in the pedal but through the car. Try replacing the near side wheel first with a good one from the dealer to confirm this.

Thank you for your comments. I have tried the brake pedal “test” and you are correct. There is not really any feedback through that.
My normally excellent Volvo dealer let me down today. All they did was to re balance the N/S front wheel yet again (4 times now this has been done!) but only told me too late for me to get them to do anything else. Its back in tomorrow and they are going to swap the wheel and see what that does. What really winds me up is when they say they have “road tested it and it is ok now” when clearly nothing had changed!

Just to finish this off for the benefit of anyone getting the same problem.
After many vists to both my Volvo main dealer and a tyre fitter, having 3 out of the 4 tyres replaced, the alignment checked and countless wheel balances where nothing cured it, Volvo finally sorted it.
They first put two different "donor car" wheels on the front, no difference. They then did the same on the rear... that worked. So they took a look at the rear wheels (apparently everyone had been concentrating on the front) and the ONE WHEEL that had not had a new tyre fitted was badly out of balance (and there are a lot of weights on there now). But its cured. I still have no idea how a rear wheel suddenly goes out of balance. But it is ok now.