Evaporative Water Coolers - Noisy, vibrating MasterCool

My MasterCool has been vibrating basically for the last couple of season. It'll start making horrible howling noises and I'll climb up my roof and tighten the pulley to the squirrel cage, oil the bearings, and inspect the belt. The horrible noise goes away (somewhat) for a while but returns and so I'm up there re-tightening again. This season I inspected the squirrel cage pulley and found that it was tweaked so I felt I solved the problem once and for all but alas the problem has come back - ugh! I've also noticed that there is a low pitch hum and the main case vibrates when the cooler is running - it just doesn't seem be balanced and smooth running. I've yet to replace the bearings because when I can't seem to detect any play up or down or side to side so I just asume they are ok. Is it a shaft problem or perhaps bearings afterall or something else all togther? Oh, I have made sure the belt is tightened to give it about 3/4" give when pressed on.