> From:    Paul Terry <paulterry@EARTHLINK.NET>
> Date:    Sun, 10 Jun 2007 17:49:52 -0500
>
> 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 know there's at least one electric fan that has two speeds.  The
higher speed kicks in as the engine gets hotter.  It could be that
you've got a chunk of fan blade missing (from a rock or a really big
bug), or maybe you've got some extra mass on the fan blade (lots of
really big bugs?).  The missing/extra mass makes it unbalanced, which
might not show up until it kicks on to high speed.

Alternatively, you might have some piece of trash stuck near the edge of
the fan, such that the blades hit it as they go by - but the noise isn't
loud enough to notice until the fan kicks on high.  Or it could be as
simple as a loose mounting bolt on the fan motor.

Matt Roberds