Could be your AIR pump going bad. This part is emissions-related and only runs for the first minute or two when you first start your car cold. The pump bearings might be going bad, or you may want to check the rubber elbow hoses that lead from pump to the head for holes.
A good test is to simply disconnect before you start car in the morning and see if noise disappears. The AIR pump is located just above the alternator.
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi