I think you guys are talking about two different noises. If it's the noise that only occurs once after a cold start it's probably the Vanos oil check valves bleeding down when the car sits for a while/overnight. We've repaired two cars this way and neither one has the noise now...
"CORRECTION: VERIFIED CUSTOMERS KNOCK CONCERN. KNOCK NOISE HAPPENS APPROX 60% OF THE TIME ON COLD STARTS. WHEN NOISE OCCURS IT HAPPENS ONE TIME AND ONE TIME ONLY. THE NOISE IS VERY SIMILAR TO A DOOR KNOCK. WE HAVE LISTENED TO THE ENGINE WITH STETHOSCOPES AND FELT VARIOUS COMPONENTS WHILE THE KNOCK IS HAPPENING. WE HAVE RULED OUT THE STARTER MOTOR AND THROTTLE MOTORS DUE TO THE FACT THAT THE KNOCK CAN NOT BE FELT AT THOSE COMPONENTS. WE COULD FEEL A SLIGHT REVERB FROM THE NOISE IN THE FRONT OF THE ENGINE. REPLACED TIMING CHAIN TENSIONERS AND RETESTED. KNOCK NOISE RETURNED AFTER TENSIONERS WERE REPLACED. STARTED TC CASE. WE WERE ABLE TO GET THE ENGINE TO MAKE NO NOISE WITH EITHER ONE OR BOTH OF THE THROTTLE MOTORS UNPLUGGED. THIS PUTS THE ENGINE INTO BASE CAMSHAFT TIMING. WE CHECKED THE OIL PRESSURE. OIL PRESSURE RAISES QUICKLY TO 90PSI AND DROPS TO 75PSI WHEN THE KNOCK NOISE HAPPENS THEN RETURNS TO 90PSI. WE REPLACED THE VANOS OIL CHECK VALVES DUE TO OIL BLEADING DOWN IN THE SYSTEM WHEN CAR SITS FOR EXTENDED PERIODS AND NOISE IS PRESENT. NOISE NO LONGER PRESENT AFTER REPAIR."