How old is this PS3? Where is it sitting?
Possibilities of cause, in decreasing order of likely hood:
@ Clogged heat sink
@ Dying cooling fan
@ Dried out, old thermal paste
@ Corrupted read from thermosistor/co
@ Corrupted temp regulation system
I'm no PS3 expert, I don't know how they're put together. But in essence, they're sophisticated computers, and all computers (especially ones with intake vents near dusty areas like carpet or shelves) need their heat sinks cleaned. F air cannot get to the vanes of the sink, it won't get cool.
As for the loud fan, it's possible the fan is on it's last legs. Cooling fans typically get louder the older and closer to death they get as the bearings wear out and the motors struggle. You may be able to find replacement fans here, or at newegg or similar sites. I imagine the fans are not that different from computer fans, but again I may be wrong.
Best of luck, hope this helps.
EDIT: the fans are completely different from computer fans, but you can get them here.