I've built quite a few systems with AM3 processors in, but there's one that is a bit unusual. It's the only one with a Phenom II X2 550, and the stock cooler fan likes to run at 3000rpm on typical Asus Q-Fan settings. Compare that to pretty much every one of the others (lots of Athlon II X2s, one Athlon II X4, four Phenom II X4s, all non-'e' editions) and the stock coolers are running at 1500rpm or less to achieve the same core temp of about 35 - 40C at this time of year.
I visited the customer using the Phenom II X2 today, and because I've been playing around with the Asus Q-Fan settings on my system, I tried a couple of things on his - setting the performance profile from optimal to silent for example, and changing the fan connector setting from PWM to DC for some reason slows the fan speed down a bit more - has taken the typical fan speed down to 2000rpm to make a much quieter system which he is quite happy with. I monitored the fan speed for quite a while after the setting change, it was stable on idle, and the fan speed ramped up to 3200rpm once the processor had been max'd out for about 12 minutes and kept the core temp below 50C. The system is well-ventilated so I'm confident that those settings will be ok.
The whole thing just makes me nervous of getting Phenom II X2's. Did I get a freaky one or something? I didn't check the thermal paste today, but if the thermal paste layer was in any way abnormal before I installed it, I would have known exactly where to look for the problem when I originally built the machine.
The processor isn't being overclocked - its idle speed is 800MHz, and its max'd out speed is 3.1GHz.