My 1985 K100 developed an infuriating rattle at 25000 miles. The rattle was a ticking sound and that seemed to come from the upper left hand side of the bike. It was worse at idle or at least more noticeable and I had visions of piston slap, cam chain slack, fuel pump failure, and other expensive problems. The local shop tried for 4 hours to isolate the problem and failed.
I finally got out my trusty stethoscope and found the trouble to be in the fuel return check valve! The only purpose of this valve is to allow you to take the tank off without spilling fuel all over. In later models, it was replaced with a standpipe that extends to the top of the tank.
To fix, you will need to remove the tank and unscrew the the valve assembly from the return petcock at the bottom of the tank. Look down into the petcock and you'll see a spring and a ball. Pry up the tabs and remove the spring and toss it away. Leave the ball and bend the tabs back. Fuel will weep without the spring in place but it still works pretty good for what it is supposed to do.
The clicking will go away and peace of mind will return.