What PSU do you have? The fan shouldnt need to be running full speed ever on a decent unit, im not sure if the fan on my EA500 ever turns on, i never hear it.
The RPM of the fan is important, but the design of the fan has a much larger effect. For a PSU since its such a small enclosed space you would need a fairly high CFM fan to maintain the pressure and force the air around the heatsinks.
What was the RPM / CFM rating of the original fan?
What is the RPM / CFM rating of the fan you're using now?
CFM = Cubic feet per minute of air moved by the fan. Also can be rated as cubic meters an hour m³/h
The CFM isnt exactly the best way to measure it, as 90% of companies over exaggerate by about 2:1 on the fans. Also, what size fan was it before, what PSU do you have, and what fan did you have before? You might want to buy a higher pressure fan, like made for dense heatsinks. This: