I occasionally get a strange noise form the small water pump mounted on the left hand side of the engine compartment. I think it is on the way out. The car is a 98 ML 320 with 55K. Please confirm the follwoing: -

1) What is this pump for
2) How long should it stay on, after the engine is turned off
3) How much do new ones cost?

The original purpose of these pumps was to make sure that the heater stayed hot when at stop lights, but not they are used to pump water thru the engine even with the key off. This allows the engine to TRANSFER heat into the water more evenly. According to MB the pump should run for 10 minutes every time the engine is shut off.
Part number is 001-835-60-64 & list price is $182.