Noisy evaporative (swamp) cooler (long)

FWIW, I live in the far SW corner of Arizona.
Probably lots more than you really cared to know.
The cooler makes noise when running. A sort of loud "humming". It's not the water pump. A few days ago I tried changing the blower belt to a new one. The noise immediately went away - for several days. Unfortunately, it is gradually returning. To me this indicates the motor and blower bearings are ok?
The cooler runs fairly quietly when first turned on, then gets gradually noisier as the day wears on. I generally turn it on about 10 AM and off about 10 PM.
I have a new blower pulley but am reluctant to change it. The blower shaft is hollow and I'm afraid if I use a puller I'll crush the end. The pulley is very firmly attached to the shaft because of the wet environment in the cooler. Lots of WD 40 for a few days preceding an attempt?
The motor pulley is an adjustable one, very handy for setting the belt tension. Could this be the problem?
The blower pulley is the original and has not caused any problems in the several years I've had the cooler. The motor failed and was replaced with a new motor and pulley at the beginning of this cooling season, probably around April. I've used other 'identical' pulleys on motors in the past with no noise. I don't recall that the noise started soon after I replaced the motor, but my memory at age 73 is not the most reliable.
I could dismount the motor and replace the motor pulley with a new adjustable one but at my age and in the early morning heat - 80+ - that is a job I'd rather not tackle. I could live with the noise the rest of the year and replace the motor pulley and belt next Spring when I prep the cooler for another year. The cooler will be probably used until well into October.
I keep the blower bearings lubricated.
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