engine rattle

Hi,
I need a kinda urgent enquiry and am hoping that you could help.
I noticed that my a/c had stopped working on my way home. On getting home i noticed that the car had started to make a rattle noise. Its an inconsistant rattle/grind but more a rattle that appears to be coming from the compressor but im not sure. It makes the noise at idle, or when i rev the engine but its harder to make out when the engine revs.
It drives as normal. It makes the rattle with the a/c switched on or off. The belt feels ok, if not a little cracked on the inside. The a/c fan works when it should. No bubbles in the window.

I can feel a vibration through the top of the powersteering filler when the rattle takes place.

the a/c has just been serviced a week or two ago by the main dealer.
My car is currently in a different garage for an unconnected matter (fuel modification) and the mechanic their had a very brief listen to it and he said that he didnt think it was the compressor. but the tone in his voice suggested that it was something more expensive!! Hes calling me tomorrow and i would like to be armed with some information if possible. Its a 99 4.0ltr very low milage.
many thanks

could loose torque convertor bolts cause this noise, and make it sound as if the sound is coming from the compressor?

Only during the same journey that i noticed the noise, i used 4 x 4 very temporarily just to check it out!
many thanks

anyone?
Could it be the power steering pump thats the problem. could a faulty power steering pump make a metallic rattle noise like that explaining the vibration that i feel whenever the rattle takes place?

many thanks