Not too sure about 4-matics...
but the oil pressure drop is perfectly normal. 3 bar at anything above idle, and can drop as low a 1 at idle when warmed up. 1 bar is approximately 15 psi and I believe 7-7.5 psi at idle would be sufficient.
The rumble at idle (could it be a rattle?) could likely be coming from the exhaust system. Are all the rubber hangers intact, does the exhaust pipe knock when you try to wiggle it (cold of course)? If the engine vibrates at all at idle, and the hangers are shot the pipes and/or mufflers could easily be hitting the undercarriage. Once off idle the engine might smooth out, ceasing the knocking pipes.
It's worth a check.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.