Hello, I am the proud owner of a BMW R100 GSPD, 1995. Reliable and agile (60,000km). The problem is that it vibrates around 4000 rpm, and this on virtually all speeds of the transmission. Before 4000 rpm and after 5000 rpm it does not vibrate. The bike is in an A1 state. Would anyone have any idea where these vibrations might come from? Thank you and ride carefully.