Rep Power: 0

I recently acquired an 08 civic si fg2 as a project car; the car needs lots of work and I'm starting with fixing the problems before moving into modding. One problem is a noisy input shaft bearing that happens only when the ac is on; so when the car idling and clutch is engaged and ac is on there's this rattling noise that goes away as soon as I press on the clutch. When I turn off the ac, there's no noise when the clutch engaged. The first thing that I thought of is a bad motor mount so I checked all the five mounts and they all look in good shape and there doesn't seem to be significant engine movement when car is ideling with ac is on. I'm still thinking though it is a bad mount, as is there anything else that can cause something like this ? And which mount of all five it is ? Any ideas

 
Rep Power: 518

That's a really weird association.....maybe it's not an input bearing noise, your description does not follow usual logic or pattern.

try the same situations but use a wrench to apply a bit of additional pressure to the serpentine belt tensioner to make the belt tighter (try looser too), see if that makes any difference

Stethoscope to pinpoint the source or area of the noise origin

Use a dial indicator and measure crankshaft end play.

 
Rep Power: 150

Make sure the top hydraulic mount on the right side (passenger) is not touching the rubber bump stop when the load from AC is put on engine. Should be at least a 1/4" gap or so. I have a thread to help you understand what the stop is if you do not.