MikeC wrote an article where he says the following,
There are two rules of thumb here:
1) A fan should be slowed down to the point where it provides just enough airflow to prevent overheating.
Overheating suggests to me the point at which the system shuts itself down, but I wouldn't like to run that hot or close to it.
If you cant hear the slipstream at full rpms then keep it like that, all systems are different so are people hearing and preferences. Check if its better though, in temps i mean.
Actually, I have realised the Slipstream is louder than the Nexus. It's still very unoffensive and not really intrusive, but I will want to run it slower when idle so that it only spins up when necessary. It hasn't made any difference to my idle temps, which is strange as it's pushing twice as much air, but it has brought my load temps down a bit (except the MB which is the same or slightly higher) although I didn't record the ambient temps when testing the Nexus so it's not very scientific (if I recall correctly the Nexus tests were done with the heating on and the window shut in the evening, whilst the Slipstream tests were done with the heating off and the window open in the afternoon with an ambient of around 19c).
Core 34c, CPU 24c, Motherboard 32c, T1/D1+ 24c
Prime95 Blend: Core 66c, CPU 55c, Motherboard 36c, T1/D1+ 55c
Idle: Core 33c, CPU 24c, MB 34c, T1/D1+ 24c
Prime95 Blend: Core 62c, CPU 51c, MB 38c, T1/D1+ 51c