The new fans used on the M272 and M273 engines can be very loud. The one on my CLK sounds like a civil defense siren when it winds up. The control is smart - when the A/C system demands max cold, it will run the fan at full speed. It also runs at full speed when engine temp begins to climb and ambient temp is high. For example, in cooler ambient temps, my engine temp can climb above 90C and the fan just barely runs, but when it's 90F outside, the fan comes on full blast when the engine temp edges over 85C.