Originally Posted by
Frogon
Your motherboard isn't controlling the fan properly, this happened to me when I upgraded my heatsink, the stock fan was a 60mm, and I put a 80mm in the same 3 pin, and the fan ran about 500RPM. What you can do is hook it up through the 4 pin on your power supply, this should max out the speed of the fan, and get everything nice and cool.
I think the reason stacking fans isnt good has something to do with them creating a bubble of low pressure inbetween, and that hurts the fan. But in any case, stacking them = bad.