What a strange vibration! Some weeks ago a colleague of mine and I went to an Hydroelectric power plant to perform some electrical tests on an High Voltage transformer. This power plant it's on the Appennino (a mountains'chain that goes from north to south in the middle of Italy) between Genova and Piacenza, and the street to get there it's very narrow and full of curves, in addition some days before our trip there was a huge rain storm that made fall in two or three hours the amount of water that usually falls in four months. The storm made several damages at the street that was broken or covered of mud in many places, and also to the power plant that was flooded (that why we went to perform the tests). We arrived there with our new company car and at the outward trip we didn't have any problem. During the return trip, when the street became a little more straight and we was able to speed up a bit, the car started to vibrate. The vibration seemed to be generated by the rear wheels, and it increased with speed. So we stopped the car and we checked the tyres looking for something strange. We didn't find out nothing on the outer diameter of the tyres, but instead we found out a big amount of solid mud stuck in some points of the rim (especially on the right wheels). We removed the mud and the vibration disappeared. It's unbelievable that some mud can change the balancing of a car! I really couldn't believe that it could be possible.