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.
Burning since 2008
USSC 6039 with many, many modifications to it.