If you're talking about a nForce4 mobo, there is a good chance that its chipset fan will be noisy... I, however, never heard mine (MSI K8N Neo4 Platinum), and that's with two silent 120mm case fans, an Arctic Cooling Freezer 64, a NV Silencer 5 Rev. 3, and a very silent PSU with a single 120mm fan...
No screaming chipset fans for me, but others report having obnoxiously loud ones... It seems like a case of YMMV...
EDIT: I never had any luck with Speedfan unfortunately... 🙁 If I want to slow down, I regulate it with a Fan Mate 2, or I solder a resistor on...