On my trip from Raleigh, NC to Destin, FL (about 12 hrs) with the air conditioner running, I was startled by a loud vibration coming from the front of the van. I pulled over and turned the motor off and a few seconds later the noise stopped.
I suspected it had something to do with the AC, so I turned it off and motored on. It was really hot yesterday and I could hear the radiator fan starting up occasionally and it sounded normal. But when I turned on the AC (traveling about 65MPH) the load vibration stated again after 30 minutes or so. Is there a second fan for the AC or does anyone have any idea what the problem is?

Thanks,
PAUL