I replaced the fan assembly at first, thinking that was best. Of course, that left me with a sinking feeling that I didn't fix it correctly and wrecked another spare smoke unit I have for future use.
So I went back inside and removed it. I took the stock fan impeller and filed it down less than 1/16" all around. I noticed that regular smoke impellers have more space around them in the housing. So I believe this impeller is oversized. It's such a tight fit that any imperfection causes it to rub. I have to believe that the material it's made from, somehow swelled up?
I dabbed some loctite inside and will know tomorrow how it works. Hopefully, I didn't glue the motor!