Hi Toddly,
I've used the Zalman northbridge heatsink, I'm pretty sure that's the fan you're talking about (
http://www.zalman.co.kr/eng/pr...sp?idx=71&code=014 ). I had a noisy fan as well, and this took care of it.
Most likely, the fan and heatsink can be removed. At the least, you can unscrew the fan from the heatsink, then the Zalman heatsink could screw back on in place of the fan. I ended up prying the heatsink off of the chip, it wasn't that hard to do. I believe that Zalman link has an install movie too.
Also, I removed another heatsink from the northbridge chip of another board, and it was attached to the mobo via some plastic connectors. These are pretty easy to take off, although easiest if you're able to get at them from the bottom of the motherboard. So, you might want to take the motherboard out of the case first...at least that way was easiest for me.
Good luck!
Tom