In my experience, the case fan positions on most computer cases have so little opening that the fan is working with its hands tied behind its back! The air holes are too small and too few in number. The fan operates in aerodynamic stall because the airflow resistance of the 'grill' is too high.
The following does require metalworking skills and the right tools and I take no responsibility if you get it wrong. However, it will give the fan(s) much more freedom to do the job you've bought it (them) for. If you don't feel able to do the job yourself, seek help from a suitably skilled friend. The following scheme applies to the fan positions in the computer case, not to the fan in the PSU, don't mess with that one!
First, shop around for fan grills made of chrome-plated wire with loops at the corners on the same mounting-screw centres as the fan. You might be able to get these from a surplus store or from an electronics component supplier if you have a friend in the trade.
Once you've got the grills, take everything out of your computer case and put the components in a safe place. Then, using a felt-tip pen or similar, mark a circle, centred between the fan mounting-screw holes, the same diameter as the interior of the fan casing (i.e. the tunnel in which the fan rotor spins). Cut away the sheet metal within the circle and de-burr the cut edges. Make sure you remove all the swarf and metal filings from within the case and get rid of it before you re-assemble the machine.
Mount the case fans inside the case with the wire grills (finger guards) on the outside and rebuild the computer. Your fans will now be operating far from stall, they may actually turn more slowly but will shift much more air than before. Set the machine to work as after any other build.
Best regards,
Pete W.
If you can keep your head when all about you are losing theirs - you haven't seen the latest Change Note!