Recently I temporarily switched back to an AMD stock cooler for my triple core Athlon II 425.

The myth that these stock coolers sounds like jet engines is wrong! In terms of loudness, they are audible but any non-SPCR user would find it very quiet and unobtrusive. Even an SPCR-user would find the noise level tolerable. However, the quality of the noise is mediocre. It can get a bit annoying because it's slightly high-pitched. Higher than an average hum.

The fan spins at 1300-1500 rpm and it's controlled via FanXpert in Asus AI Suite (run at 10%).

I'm guessing the 10% isn't a true linear response - otherwise that cooler is more of a jet turbine at full speeds

The AMD coolers are pretty quiet (95w and under ones) if you ramp down the settings in bios etc etc. They're nothing fancy mind but they do the job.
I did try a Gelid siberian for a 65w dual core AMD it's inexpensive and you get a decent PWM fan. Playing about a bit in bios on an Asrock budget board it works very well actually and has a low noise level and very smooth overall even at higher speeds, it's a budget alternative to the AMD ones. Again don't expect too much looks wise but you're basically paying for a fan and getting a heatsink for nothing!

I guess it depends on fan bearing ..... I got some 125W AMD stock coolers which has some nasty bearing noise. It's not terribly noisy, but it sounded uncomfortable(hum) for me despite its 1300~1500RPM.