PSU fan not spinning / speeding up, knocking

I have a Corsair GS700 PSU from around early 2011. It's been working fine, but after a recent move I turned it on for the first time at my new place and I heard a problematic noise. After a few minutes in Windows, I noticed a kind of grinding sound inside my PC, like a fan starting up, hitting something a few times, then stopping.

I checked and saw that the offending fan was in the bottom of the PSU. I turned my PC on its side, blew out any excess dust, turned it on again and ran a game. When the fan kicked on again, same thing happened. Then I turned the PC on its side again, then watched what the fan did. As I boot up, the fan starts trying to spin, but it barely reaches max revs before kicking out. It kept trying to start up to full speed every 2 seconds or so, but it never took. I removed the fan grill, blew out any dust, removed any visible obstructions (of which there were none), even turned it on and tried to give it a bit of a manual shove to try and get the fan to keep going; no dice. So I replaced the grill, eyed the "Warranty void if removed" sticker, wondering if I should dig inside it, then decided to ask the community first.

Can someone give me some pointers as to what I can do to get the fan spinning again? Should I dig inside the PSU? Any tips?