On my 2003 MCS I have a low frequency vibration at idle that I don't feel throughout the rest of the RPM range. I don't remember this from before I installed my pulley, but I've checked the mounts and can't seem to find anything.

Only changes are 1) 15% pulley 2) new air filter

I also have a hesitation in my lower RPM band now, making it a little harder to smoothly pull away from stop lights.

 

similar situation here. any update?

 

Turn off your AC and see what happens. I had that but once I added an exhaust (I'm thinking more power in general) all of that hesitation stopped but the vibration is still there on idle.