Hi, I had a Q9400 and it's a cool running chip, unless you've got some heavy over-clocking going on.
As a budget solution I like the Hyper TX3
, currently $18. The stock fan isn't wonderful but is PWM and can be spun right down to 700rpm or less at which point it's really quiet. Probably also still cool you CPU ok without needing to speed up from this speed often either. You may need to change settings for fan control in your motherboard's BIOS, usually in the system monitoring section and called smart fan or Q-fan etc, to get the lowest / quietest fan speeds. In fact this may well help with the stock cooler too!
It comes with s775 and s1155/6 mounts so you may be able to use it on a future build, not relevant to you but can be mounted all-ways round on AMD systems too, which is useful and unusual. It's bigger brother the Hyper 212 is also great value although I'm not sure how good the stock fan is.
I've also used Freezer 13
and the stock fan is very quiet at minimum speed with good cooling.