Have come across a number of sets of CV joints doing this but not displaying the usual full lock symptoms that Gavin is referring to. The one thing they all had in common is that they had done around the 110,000 miles mark when it started. Generally it started at 80mph and as it became worse the vibration would cut in at lower and lower speeds. The only evidence I found was to look at the bearings under a magnifying glass and you could see the pitting in the surface. Only other related symptom with this Ive come across is very slight play in taking drive up from stationary.
Have come across a few friends that swore blind the noise was coming from the front and it turned out to be either a rear wheel bearing or square rear tyres! Swapping wheels front to rear is a quick test to confim the latter!!
Square front tyres are normally noticeable at much lower speeds than 40mph in my experience.
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