Posted a number of messages here about a tapping/knocking noise near the front of my 540 engine, audible at idle and only after 10-15 minutes of driving. Brought car to JMK in Springfield, NJ, where the problem was diagnosed as a VANOS failure on the left-side of the engine. This was not an easy diagnosis-- intermittent noises never are, and I could easily have been blown off. My hat's off to Tony, Jack, and Andy for their help. I post this message as well to alert others,as it seems BMW is aware of other similar cases.