Mounting sleeve bearing fans in a non-vertical orientation will usually result in more noise and reduced lifetime.
As best as I can tell, some bearing mechanisms don't like being horizontal and the oil drains out. The fan develops a whine, you push the blade off the motor hub, add a drop of oil, and put it back to work.
It could also be that the fan I've got is a Silenx and they're just crappy fans. I wish somebody had told me their figures were complete bullshot before I bought 3 of them