What about the Prison level makes my GPU fan spin so loud?

[MASKOAA]

i5 2500k
SLI GTX 570 - Game only uses one b/c thats all it needs really
8 gigs ram

Fan spins up in the PORTAL MAZE area earlier in the game and the whole time I'm in the Prison area but the prison area doesn't look very demanding maybe someone could clue me in? Fan's basically blazing on this section. I have it vsycned to 100fps b/c my 120hz monitor is set to 100hz.

[Disasterrific]

I get a bit of slowdown in those areas - it's a matter of multiple light sources and long draw distance.