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The left hand side of this image shows the slight inset of the base of the heatsink. This part should sit over the raised part at the back of the socket.
Also have you attached the temperature sensor that comes with this heatsink or do you have it manually set to the lowest speed?
Also something to bear in mind is that there are 3 different versions of the XP core (well 4 if you include the new bartons
). Original XP Palomino core, Throughbred A and Throughbred B. The later cores run cooler, so when you expect 20C operating temperature you might be getting a figure for a later core revision than the CPU you have.
Also I noticed this on the thermaltake website:
Temp. control, fan speed auto control setting:
1300 rpm at 20 ºC~ 4800 rpm at 55 ºC
Note that this is the speed the fan will run at when the sensor reads 20C. I don't have any experience with this heatsink so I can't say what it runs at without the temperature sensor being utilised.
Or do you have the jumper attached to run Full Speed 4800 RPM?
Just a few things to consider.
Personally I'm running a 2100+ Tbred B @ 2.171Ghz with a ThermalRight SLK-800 and YS-Tech Silent 1950rpm (20dBA!!!) fan. That paired with my other cooling has the overclocked chip sitting at ~35C idle and never peaking over 45C full load. ThermalRight heatsinks are considered some of the best, if not the best heatsinks available. One to consider if you are changing HSF.