Chumster

Morning everyone! I've been reading the forum for a while and this is my first post. I just bought a (brace yourself) 2000 XJ and have a rattling issue that I hope you can help me with.

The rattle is consistent at about 1100 rpm when i'm in gear only. If I'm in neutral it doesn't do it. It kinda makes the sound like an air compressor until about 1200 rpm and then stops. I was thinking it was coming from under the jeep by the transfer case but i finally found that at that Rpm the a/c compressor is shaking like no ones business. I can't tell if it is the compressor itself that is causing the shake or if the shaking is coming from somewhere else and just making the compressor flop around.

From what i can tell, the electric fan is not rattling but the sound could be coming that direction? The frequency of the rattle is 280 hz if that helps for anyone (I'm majoring in mech. engineering so I'm just learning the properties of vibrations...) from this forum, I discovered yesterday the flaw with the cylinder head in this year, could the vibration have anything to do with that?

I'm at a loss, thanks in advance for your help!!