But I can't afford a new HSF at the moment
Anyone want to donate one

, LOL!!!
The CPU Fan is definitely adjusting it's speed to the CPU temp, just did a 3DMark06 and you can hear the CPU fan get louder the longer the tests run.
Only thing I can do is mess about with a duct and see what happens until I can afford a new HSF, it seems most people think that's a better option!