High speed periodic vibrations

[TB2]

So I'm taking long trips through Germany quite regularly and I like to drive somwhere around 140-150km/h (90mph) which my turbo does just fine, but I'm having slight problems with vibration and I'm wondering where to start looking. The odd thing is that the vibrations occur periodically around every 3 seconds for 1 second. Uhm, how to demonstrate it in an online forum... it goes like:

"wrrrrrrrrrrrrrrrrrrrrrrrrrrRRRRRRRRRrooooom, wrrrrrrrrrrrrrrrrrrrrrrrrrrRRRRRRRRRrooooom"

...at constant speed on any sort of surface, no matter how smooth. It must be some sort of interference, maybe between engine noise and something else. During the vibration peak, the steering wheel and dashboard flex quite heavily. What I already know:

1) The wheels have been balanced by a professional with these tyres fitted.
2) I changed the break discs and pads and the problem was present before that - no difference - so I assume the discs are fitted properly. (Also there's no vibration from the brake pedal, indicating unbalanced discs).

I'd love to get rid of this problem. I *think* that the rear wheel bearings might be knackered, but I'm not sure how to check. But they really sound kinda "crunchy" when I turn the wheel with the rear on a carjack. Could this be the sole reason? Is it hard to change them? (I heard scare stories about special tools and some heating/cooling trick...)

I fear it's a combination of different small problems that accumulate to this interference pattern. Any more hints?

Thanks


PS: btw I alway love to repeat this: I'm astonished by the fact that the BX doesn't rattle at all. So many old cars sound like they're about to fall apart on the highway. Even though the Dash bounces quite strongly, there's no rattle from the vents, the seats or any other bits. I really like this

PPS: And who at citroen decided to use 5-edged nuts on the front calipers??? Took me a minute to even realize *why* the damn socket wouldn't fit. (I wanted to adjust the angle because the old pads were unevenly worn....)

[Vanny]

Its not something simple like the windscreen seal is it? I found when mine started to lift that it would vibrate at high speed once a cross wind had caught the edge of the seal. The location of the noise wasn't at all obvious until it became worse.

This does assume that the dash and steering wheel vibrations are simply coincidental!

Changing rear arm bearings is relatively striaght forward. My record is three hours start to finish.

[TB2]

Mhh, not the rear arm bearings, I meant the rear wheel bearings. I changed the rear left arm bearing already and it was in really good shape so I didn't change the right one.
But the wheel bearings might need changing as well?

The windscreen seal is in good shape.

[scarecrow]

Could be worth getting the wheel balance checked again. It's not unheard of for the weights to fall off...

Engine mounts - has a rubber gone? I had a mount fall off on a R4 once and it was more of a power take-up/off knock. I never drove it at any speed though. (Incidentally, I wouldn't recommend making a new mount from cherry wood - much vibration).