Hi,
I finally got fed up with listening to the rattling noise from my GW Instek GPS2303. The rattle showed up after it had been running for a while at low current output, and at low fan speeds.
Initially I thought that the rattle must be from the fan bearings and took it apart in preparation for finding a suitable quiet PC fan, then I realised that it is a 24V fan! The bearing actually felt fine, it's a double ball type with no slop, the spec. indicates that it is on a par with the quieter ones anyway.
Next I investigated the drive waveform - a 555 generated rough PWM with fast rise time and 65Hz repetition. I could feel the vibration in the fan rotor. The only thing filtering the fan supply after the switching transistor is a little 10uF electrolytic (C56) which, as far as I could trace, is straight on the switching transistor output (certainly straight across the fan), so not having a happy life. At low fan speed the PWM beats with the fan comutation too, causing more repetitive rattle. The 'rattle' is actually structural resonance in the fan, coupled into the case.
I didn't want to take the power board out, so I chopped into the fan lead to add an RC filter. After some experimentation I've settled on 10R 1W and 220uF (100uF works nearly as well), and fixed them to the rear panel. The result is that the PSU is now silent apart from transformer hum, the fan cuts in smoothly and quietly. I was initially worried that the sharp PWM pulses were needed to start the fan but it actually seems to cut in a bit earlier, and is running slowly by the time the heatsinks hit 40'C. The voltage drop across the 10R resistor (1V at full speed) doesn't affect full speed operation, in fact it seems to run better with the smoother supply. I haven't managed to top 55'C on the heatsinks with both outputs shorted and set to 3A. I've also snipped out C56 to reduce peak current through the switching transistor (Q51) which gets hot enough anyway.
There is obviously fan noise and 'whoosh' at higher speeds and some resonance into the metal structure (I may investigate resilient mounts), but at normal loads it is now virtually silent.
I hope this helps someone. Photo attached.