Since we got the coach, the driver's side (only) windshield fan has vibrated continually and annoyingly, beating time against the powered sun visor screen mount given half a chance.
As my wife has been learning to drive the coach and gained skill, we are both much more relaxed, and I'm able to look around as she drives. I noticed that the passenger side fan was nearly motionless. Hmmmm....
On further inspection, I found the bracket that attaches the motor/fan assembly to the base was installed between the fan guard and the motor on the passenger side, and on the end of the motor housing opposite the fan blade on the driver's side. Taking it apart and putting the bracket on the other side solved the problem. Life is quieter and less distracting now!
Here's a pic of a similar fan assembly on Amazon:
The third pic shows the bracket I'm talking about. The second shows it *not* attached to the motor housing end at the back of the fan.
How the prior owners lived with this for 89K miles is a different question!