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.