This has been an issue for me because I use my SC1420 as a DAW with ProTools (it works excellent in that capacity) and the fan noise, the VARYING fan noise, is a huge issue when trying to critically listen to music. My model has one cpu but I ordered it with the two cpu fans. I had earlier disconnected the unused (top) fan but it didn't help much.

The Nidec fans that come with server are among the strongest 92mm fans available and push around 150 cubic feet of air per minute. That's more than my bathroom fan moves, and explains the noise. I recently replaced the two cpu fans (I have one cpu but got the dual fan option) with Vantec Stealth fans (model SF9225L) from newegg.com. I first installed one on the cpu but the cooling fins ran warm so I installed the second in the second cpu shroud (just to exhaust air from the top of the case) and that did the trick. There's still a Nidec in the side of the case (pulls in air over the hard drives) but the system is MUCH quieter and the cpu cooling fins never get warm. The Vantec fans are only rated for about 30 cf/m each so I don't know if this would work for a dual cpu system. There are other 92mm fan options, but this is what's working for me.

 

Oh, the Vantec fans came with a 3-pin connector. To install them I just pulled the black 5-pin connector shrouds off the motherboard (carefully--it's just a friction mount) and the 3-pin connector went right onto the bare leads.  Hope this helps...