Over the past 2 months I have noticed a engine shutter when my 300SDL comes down to idle. The idle is about 650 rpm, the engine has good power, and I do not burn any oil. I have notice a decrease in my mileage to 25 rather than 30. This may be due to snow tires and winter fuel. The mechanic I have been taking the car to (who so far has been great) said the problem was probably the idle governor and did not recommend any repair as the shutter is no big deal. Any ideas??