CPU fan sounds like a freight train, at a guess, because it is motherboard controlled, and the cpu is too hot, so the board is increasing speed to max, trying to cool it down.
Cpu be hot because you have a nice caking of dust insulating the heatsink.
The other fan probably generating noise, will be your power supply, which is almost certainly, internally coated with the same insulating layer of dirt.
And the gpu, just so you didn't think you missed anything
Step 1: Take case outside after disconnecting cables, and gently, but thoroughly, blow it out with the compressed air you mentioned.
Step 2: Judging from case, do step one twice
Remember to try and shoot some down through the gpu and psu, and get those blown out as well.
You should notice a massive difference at this point.
Good lord man, the computer gods are giving you the evil stare at this point
Dust builds up because of lack of filtration in the cases, combined with charged surfaces, even if it isn't much. Charge attracts dust, etc.
Basic solution is to open up the case every few months and give it a thorough cleaning. Your fans will last longer too.