Yey!!
Great answer, since the intakes have valvetronic the intake valves are at minimum lift at idle (and at shut down) which doesn't allow any spring pressure to push the oil out of the HVA's.
My only answer to the short trip driving causing it more often is that maybe they don't get completely bled of all the air before the engine is shut back down again, causing the noise to come back immediately.
The two solenoids on the front of the engine control the oil pressure to the Vanos units mounted on the front of the camshafts. By varying the oil pressure to the Vanos units the DME can advance/retard the camshafts for variable cam timing. I'm sure you can guess, but one is for the intake cam and the other is for the exhaust cam. This system is completely seperate from Valvetronic.
Anybody up for another useless trivia question? Why do none of our engines have EGR valves on them (exluding Diesels)?