Been playing with many different fans with this issue as I wait for my DMM to come in....and positional noise differences seem random.

What I've observed is that there's no trend with sleeve bearings getting significantly worse when the airflow points straight up/straight down. Maybe this is a long-term issue? I have a couple fans that make a bit more extraneous noises when blowing sideways...and a couple that make a bit more noise point up/down. No real statistical trend on any of these fans that I'm noticing. None of these fans have been run long-term, however.

All my official testing is with fans point downward. All subjective noise comments are as well.....so take those for what they're worth....if a fan is extraneous-noise-free when I test it pointing downward, it really is (I guess...lol, I really honestly don't have any sort of trend here to be sure...heck, the Yate Loon sleeve bearings make no extraneous noise in any direction). Even when I hear a low click, the frequency of occurence is much too low to be a rubbing issue (I think)...and the low clicks I hear are practically inaudible....the dBA meter doesn't notice them, they're just a different noise.