A/c ??

I have a 01 XJ sport and I'm experiencing the most annoying thing in the world.
Okay here it goes. Whenever I turn on the A/C and I'm at a light it vibrates like all hell.
What do I do to get this to go away like before?
Is it as simple as turning the idle up a couple of hundered RPM'S?

Sounds like a loose serpentine belt. As you increase the speed, the harmonics of the belt increase making the vibration less severe.

Just a thought.