1998 Vauxhall Vectra Excessive vibration at speed.

Shakes or Wobbles problem
1998 Vauxhall Vectra Manual

Vauxhall Vectra Estate 2.0 LS. Have recently noticed that my car vibrates (even shakes) when driven at speed. As a general rule this occurs between 70-75 mph but has occured at lower speeds (65+) dependant on road surface. Once I exceed 75/76 mph the problem geberally disapears and ride is fairly smooth. It feels very similar to driving over rumble strips, though much more rapid. At present, the vehicle is no more difficult to control when shaking occurs, however it can be extremely uncomfortable. At first I thought that it may be the allignment or tracking, but I have not experienced any steering wobble or the vehicle pulling to one side. At roughly the same time that this problem began to occur I also started to notice a knocking from the rear of the car when turning right at speed (such as on a roundabout) is it likely that the two may be related?
Being very much a novice when it comes to mechanical matters, I would appreciate some guidance before visiting my local mechanic, any thoughts as to what could be the matter?