Why is my fan... well vibrating/rubbing against its own casing?

Recently (and quite suddenly too/out of the blue) my macbook's fan started to make quite a lot of noise. There's quite a few questions about fans making noise here, and I've read through them.

I decided to open up my macbook, the fan was full of dust - I cleaned it out as well as the ventilation vent, out it back together and turned it on. No change...

I then took it apart again, this time I left the back off and watched the fan spin (probably not the safest thing... But I wore rubber gloves). Same noise again...

This is the interesting part though... After taking the fan's top casing off and turning the computer on again - the noise had gone and the fan spun fine. Putting the casing on again brought back the noise.

My question is... Does anyone know why or what could be causing this? I can't see anything that could have changed so suddenly in the top casing... Unless maybe the fan has moved up on its axes?