Engine vibration

I'm having some issues with my 07 mustang v6, It just started today, When I'm at idle with the AC on, the car engine starts to shake. Its starts shaking as soon as the electric fan kicks on, the car has about 84k miles on it, I also just had my alternator rebuilt and installed this past weekend, all the belts are tight, what do you guys think it could be?

David.

Loose or misaligned alternator?

Its not loose,

motor mounts?

if it were the motor mounts wouldn't it by shaking all the time not just when the fan tuns on,

Possibly, but if it's just one and the doing of the fans irritates it or something. Idk. Just throwing that out there. What about a harmonic balancer?

here is a video i made today, you can here the vibration sound at 11sec, right before the fan kicks on,

http://youtu.be/ImP5N1MbFTo

Could it be the fan itself? The loud rattle just as it kicked on made me wonder if the fan is bad, or the blades are hitting the shroud somehow (or something else that got loose)

possibly, it sound like plastic rattling.

Something else that I just thought of - take a look at the crank pulley/harmonic balancer and make sure it isn't coming apart. Several 4.0's had them come apart before. I marked mine with a white spot of paint so I could check it when I was chasing a noise down (mine turned out to be a mis-alinged idler pulley but it was a chirp not the same sound you are getting)

Something I saw on the video, Check your Knock sensor it appears to be unplugged It's on the Left side of the engine, it appears that the plug is just sitting there, maybe since it's not pluged in the PCM is not picking up a signal from it causing the engine not to idle up a tad when the A/C is turned on, causing more vibration, Mine is going through the same problem... Please post what you find out....

I check everything and nothing was unplugged, But I did happen to notice i was missing 2 bolt from my coil pak bracket, i was able to replace one, and the rattling noise was gone,

your fan is turning on into and not starting at the low position, mine was doing the same thing, It's the fan has no low speed, it automatically goes into high speed, from a dead stop. It seem that these fans have a fail safe to the high speed. I changed my fan and the vibe went away... good luck