The current spec after the BMW change notice dated (1982?) is .008" exhaust and .004" intake. I've been using .008" EX and .006" IN for years. I'm going to try .008 and .004 this next time around and see what happens.
Also there may be too much up and down play of the rocker arms on the shafts. The should be only about .002" of endplay between the rocker arm and the post at the end. If there is a bunch of play here the rocker arm will jump up and down "tapping" on the posts.
Too fix:
Look at the supect rocker arm's push rod. If the edge of the pushrod is really close to the wall tunnel, then loosen one of the two head bolts on that rocker so that when you sqeeze (coming next) the post together it pulls the pushrod away from the wall.
You need three or four hands for this next bit.
I bought a huge expensive monster set of plyers from sears with jaws that could hold the posts of the rocker arm between them just for this purpose.
Check and adjust:
To check:
First rotate the cylinder so that the valve is fully closed and the rocker arm is free. Put the .002" feeler gauge between the post and the arm and slide the arm up and down.
If there is play with the .002" feeler gauge in place then to adjust:
Keep the .002" feeler gauge between the post and the rocker arm. Put a torque wrench on the loose nut, and put
some light gentile (think snug) pressure on the posts with the GIGUNDOUS set of plyers to squeeze them together. When you can just move the feeler gauge, tighten the posts headnut with the torque wrench. Check the valve clearence again when done.
Remeber if you clamp down hard with the GIGUNDUS set 'o plyers then you will wear out lots of expensive parts long before their time. There needs to be some play just not very much. When you are done the rocker arm should still move freely.
Repeat for the other three rocker arms.
Go out and ride it.
Also when you took off the valve cover there was oil there right? If the head parts are not wet with oil then you have another problem.
