jcoop, sounds like hydraulic lifter(s) tapping. What is the service history of oil changes and brand, oil viscosity. Check the Owner's Manual, but that engine should probably be running Mobil 1 0W40 oil with changes at least every 10,000 miles. I use Mobil 1 religiously and perform oil changes every 7,500 miles on our MB vehicles.

__________________
Fred Hoelzle