Rattle or grinding noise when AC is on
 

When I turn on the AC the secondary fan turns on and sounds fine, but then a few seconds later there is a grinding or rattle that i can't seem to pinpoint. It is only present when the AC is turned on. I can not hear it inside the car very well, but i can feel the rattle inside the car. I though it might be the belt driven fan but I am not sure.
Does anyone have any ideas?
Thanks

Relating to AC, I can think of 3 different things. 1) AC compressor and clutch, 2) Secondary fan and 3) possibly the main fan clutch.

There usually is a delay on secondary fan once you turn on the AC. See if you can feel the rattle before the fan kicks in. If the rattle is constant with no relation to the secondary fan being on, then it probably is you AC compressor. BTW, the secondary fan is electrical.

And the third option, the main fan and fan clutch... Fan clutch is temperature triggered and only engages once the clutch mechanism reaches certain temperature. So if the rattle is sporatic, it may be the fan clutch. Check the fan clutch for any visual sign of wear, and hand turn it so see if it wobbles.

Fixed!

The bracket that connects the ac compressor to the engine block was damaged.
One of the bolts snapped and that placed more stress on the bracket.
The bracket was changed and there is no rattle when the ac is on or off.