On Tuesday, June 23, 2015 at 2:08:58 PM UTC-4, Dave, I can't do that wrote:
Hi All,

Neighbor called me over to look at his new swamp cooler as the motor is making a lot of noise. We took the fan belt off and the noise level didn't change at either speed. It's a loud humming droning noise that seems to be about the resonant frequency of drywall-walls as we can hear it throughout the house.

We mounted the entire cooler on strips of foam rubber so nothing is touching any thing solid to transmit the noise. Still it persists unabated.

Is there any point in pulling the motor apart and seeing if we can make it hum less?

If so, what would we be looking for?

Dave

It sounds as if the motor is out of balance. Does the motor drive the fan directly?

Anyway you might put a mark on the motor shaft and then give the motor a spin and see if it steps in about the same place. That would indicate the motor is out of balance. Then add weight to get it so the motor has no preference where it stops.

If that is not enough improvement , Take a look at " Orthoganol balancing - twinwheel grinders " back in 09. It is a really good thread started by Jim Pentagrid. He goes into how to use a small computer speaker as a vibration sensor, And how to do a really good job of balancing.

Dan

Dan