So I posted a thread about this, and nobody has responded.
Anyway, I think some fans running "upside down" will make more noise than usual. I have a top mounted JetFlo and it's noisy as :), and annoying as :). It's kind of a "wobbly" noise or a "humming" noise.
I'm probably going to set all case fans to silent mode and CPU fans to normal mode and see if it fixes it.
Again though, I think most fans are made expecting to be run in a certain orientation.
But i'm surprised so many people here seem to miss the point as to why a fan might make noise. Some noise people are describing are not normal operating fan noise, rather noise from a fan that is out of balance.
amirite?