one of the main reasons to buy the ps3 slim was the power consumption and shrinked cpu/gpu diode, therefore cooler and more silent hardware. additionally it received this amazingly quite, improved and bigger fan upgrade. but a noise still remained in a super silent environment... the vibration from the fan caused the heatsink and therefore the case to emmit this buzzing noise.
once i had the security torx 6 screwdriver i could open the ps3 slim easily and fix the "problem". basically i just put small pieces of a bit thicker household rubber between the screws, fan and heatsink, therefore reducing the vibration transferred. see photos.
well, once you remove the fan you notice how incredibly silent it actually is. the problem was to find the right amount of force applied to the screws and therefore the right distance for the rubber pieces. when too lose, the top case of the ps3 slim touches the fan and causes another annoying vibration, when too tight it's like without rubber at all.