Originally Posted by Nighthwk12
I'd check you engine mounts
Originally Posted by tyson
Iac sensor under the throtele body clean and try before replaceing
A bad IAC Sensor would not cause an engine vibration. It would cause a rough (bouncing) idle. Also, a bad or gummed up IAC sensor would cause that symptom regardless of whether or not you're in gear or neutral. The fact that he'* having this problem in gear and not in neutral is a telltale sign that his engine mounts are bad. When the car is in gear, the engine and transmission put a force on the engine mounts to pull the car forward. When in gear, that force isn't there anymore. If the engine mounts are bad, the engine'* vibration transfers to the car'* body instead of being absorbed.
2005 Pontiac Bonneville GXP - For Sale ----- 1995 Buick Regal GS(X) - L67 madness
2000 Buick Regal GSE - Stock...for now
----- 1987 Jaguar XJ-* V12 - 12>8