Will,

Did not mean to confuse you. The mount issue is not dependent on the idle speed change from drive to neutral when the car is stopped. However, when the car is idling in neutral there is very little torque (some oscillating torque, but no real load) being absorbed by the mounts. Most of the load on the mounts in that condition is vertical, resisting gravity trying pull the engine out the bottom of your car.

When you put it in gear, the transmission tries to make the wheels turn, and the brake pedal being depressed to keep the car from moving opposes that. But the torque generated by the engine, being reacted on by the transmission and the brake, causes the engine to try to spin itself around the drive shaft. At idle this is a relatively low force, but it happens at a relatively low frequency. The motor and transmission mounts prevent that, and one (the passenger side) takes a compression load due to torque in addition to the vertical load from gravity, while the other gets slightly unloaded by the torque. Consequently, the mounts get exercised differently in this condition, and you can get a different response, especially if the mounts are getting worn out.

The added load on a worn mount can cause it to "short out" or compress enough to act like it is not really there, allowing more engine vibration to make it to the car structure. It can also change the response of the mount enough to make the typically small physical oscillations of the motor and transmission become exagerated at specific frequencies (lower frequencies are usually worse for this, which occur at idle speeds).

It sounds like changes in idle speed are not your problem and that it is more likely the mounts themselves. Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)