I took My E30 325i Cabrio into the smog shop and the guy started his procedure. When it came to the part where he revs it at tests the timing/smog at a certain RPM between 2 and 3 thousand RPM a sound started to emit from the engine I had never heard before. It was a fast nocking sound. I asked him what it was and he shut off the car and started it again and reved it up to 2000 then 3000 and so on the sound started at 2000 and stopped after 3000 rpm. He told me it wasnt a good sound. ( I could have told him that) . I have stopped driving it because of this sound.
anyone have any ideas what it could be? the part that puzzles me is that it only makes the sound between 2000 and 3000 rpm.
any help would be greatly appreciated