In vent mode, the water pump does not run. In cooling mode, it does. But you hear the sound in both modes. That probably rules out the water pump.

Does the blower have more than one speed? Does the sound occur on all speeds if it does, or just one certain speed?

How clean are the pads? Is there a buildup of mineral deposits on the louvers where air is drawn into the pads? Have you had the thing serviced regularly?

 

It happens at all speeds, but it is MUCH louder when it's on high. Every year we replace the pads, drain and flush it to no effect.

 

whirring noise

For the last few years, whenever the swamp cooler (roof mount) is turned on, whether it's in vent or cooling mode, it makes a very loud huming/whirring noise (kind of like an air raid siren that's very low pitched, or a vacuum in the other room). It supposedly started the year after installation. The air blows nice and hard.

Is it just a poorly designed swamp cooler, and should we look for a new one? Would a swamp cooler make a noise like that if it was set up improperly? Maybe they put in a motor that was too large? Could they have wired it up backwards, and if so would the fan blades spinning the wrong direction make a noise like that? It replaced a previous swamp cooler, so the ducting and mounts did not change, just the unit and everything inside.

I'm just looking for some basic advice, if the swamp cooler itself is poorly designed we'll have to get a new one installed, but if it's something else I'd like to get it repaired.

This is typical of a belt that is tight and/or bearigs that are dry. Take the belt off and see if the blower will spin freely by hand and stays spinning for atleast a minute or more.

 

Turns out there was a flap of metal that directs air flow that was too close to the fan blades, they just had to bend it a bit and problem was gone.

 

Cool; cheap easy fix! thanks for the update.