I already have the inexpensive heat sink/fan, so according to your
suggestions a fan controller or one of various
voltage-reduction modifications seem worth pursuing. Can you elaborate on
that?
But does it use an 80mm fan?
When the heatsink is very large (large surface area) and likewise the fan
large too, it can run at lower RPM than a smaller fan or on a medium-sized
heatsink.
It is possible your heatsink fan's speed could be reduced, but I cannot
predict the resulting temp... maybe cool enough, maybe not.
Since I can't predict how well it'll work for the 'sink you already have,
I'll suggest the cheapest solution then if it's not good enough you're
only out a couple bucks.
Here is a 3 pin to 4 pin adapter, so you can connect fan to it, then it to
a power supply plug.
http://www.svcompucycle.com/3pinto4pinad.html
For a single item it may be cheap to use their USPS "first class"
shipping, which should be under $1. First class shipping might take up to
a couple weeks though, at least that was my experience last time I ordered
something small from them.
On that adapter you'd want to do the "7 volt mod" which involves swapping
the plug position of the black wire connecting to the fan plug, with the
red wire. You'd want to use a paper clip, jeweler's screwdriver or
whatever's handy (unless you have a molex pin extractor) to remove that
black wire and red wire, swapping the two at BOTH large plugs, not at the
fan plug. Result is that fan uses 12V as positive, 5V as negative,
potential difference is 7V so fan spins @ roughly 65% of it's former speed
but potentially less than half as loud, maybe even inaudible but that can
vary per fan.
If my desciption of the modification isn't clear, a Google search for "fan
7V mod" should turn up some guides.
Other alternatives might be soldering a resistor in series on the fan
cable's lead, or a bunch of diodes, or buying a fan controller (available
from most larger online merchants) for about $10, though the whole
replacement heatsink is about the same cost.
Keep in mind that your current fan, since it seems to be spinning at high
RPM, is wearing fast than it would at low RPM. Typically a ball-bearing
fan will start out at a given noise level, get a bit louder during the
first few months, then stay at about same noise level for several years,
until it begins failing. If yours has been at a constant noise level but
now is louder, might be time to replace it.