I have been driving this car for three years, and have put about
30,000 miles on it in that time. it was a bit carboned up when I got
it, and I think the previous owner drive it gently- too gently. It
uses very little oil and is a dream to drive.

I recently did two jobs on my 1990 245 with just over 200,000 miles.
First I replaced the timing belt and cam seal. The leak was barely a
dribble, but it was nice to get it done. The idler pulley was also
replaced.

The other job was to stop the annoying exhaust header pipe rattle. At
about 2,000 RPM the inner and outer layers of the exhaust header pipes
would rattle annoyingly, very much noticeable inside the car. I
drilled two holes in each pipe, about 25-30% from each end, and
inserted short sheet metal, self-tapping screws. Once the holes were
tapped, I removed them and cut them off so that they would protrude as
little as possible into the exhaust flow. Rattle completely gone.

Since that time the car is running noticeably better- smoother, with
more power, and better low-end torque. This is not just a matter of
some psychosomatic reaction to the work I did

Here's my theory: I do not believe that it was the timing belt. The
rattling of the pipes, even at times when inaudible, created some
frequencies that were picked up by the anti-knock sensor which was
retarding the timing a bit. Is that possible?