psu cx 500 issue

hi, i have this psu from december of 2011, and im happy with it

but from some week i have some issue, the psu randomly shut down...

i checked all cable, reconnected al cable. temperature are ok, cpu: 25-27degree at idle, 53-55 at full load.

when i go close to the psu i hear a little buzzing(?) just a little.



the shut down is totally random, idle or full load.



thank you.