vibration when the AC is on

Hi, to all just wandering if anybody has any idea what this could be. here's the problem when my truck idling with the AC on write before the fan kicks on i feel a vibration and hear a humming sound / whistling sound but I cant figure out where its coming from I cant hear it from the out of the truck only from the inside 00 century with a 60 series in it any help would be greatly appreciated

 

thanks for the reply this sounds like it could be the problem but a air leak from where?

 

10-4 i'll start with this and see if this is the problem hopefully it is thanks