Keep the spare fan when you get it and leave the original in place. I had the same problem and got the spare fan. The sound went away a week or so later and the original fan is still humming along in it's fifth season. It's quick to replace, so if you have to do so in the middle of the season it won't matter much. 20-30 min. job.

It is as if some just needed to say the problem would fix itself. I burned last night on Pr-3 for an hour and Pr-2 all night without the Room Fan making any extra noises, clunks, or grinds.

Whatever voodoo you passed on cornpyro, thank you.