[Mart]

I will mention something that happened to me. My roadster had a terrible vibration/noise that was road speed related. I ruled out everything on the motor/input side of the gearbox side of things and was preparing to pull the motor and gearbox and tear down the gearbox. I was on a test drive trying to locate the noise, and when it kicked in I was reaching for the ignition switch to kill the motor so I could rule that out, and accidentally touched the dash near the speedo. The speedo was vibrating at the exact frequency of the horrible noise.

I disconnected the speedo cable and hey presto the noise stopped immediately. The speedo must have a dry bearing or something, I have never taken it out to investigate.

It's a thousand to one chance that this is the problem in your case, but an easy one to check and rule out.

Mart.