it's a ticking noise, like if you drop a screw on the aluminium body. happens once every 1-2mins. cant hear it while it is on load, not sure if it's there or i cant hear it.
but it's definitely there during idle.
i've spent 2-3hrs looking and any loose screws, wires, fans,etc nothing seems to be out of the ordinary.
though i seem to be missing 1 of the nuts from the casing when i was dismantling it but the noise happened before i lost it.
http://youtu.be/m5VXSCVYoi4?t=45s
recording of the noise.
happens around 47sec mark.
If it's not the PSU then it has to be one of the fans. Maybe the fan blade is hitting one of the plastic supports, and you can't hear it under load because the fan might shake less when it has more angular momentum.
Since you don't hear it under load, one way to deal with it for now would just be to run the fans 100%, you can do that with in "--avalon-fan 100" or by lowering the max temp to something that'll keep the fans running 24/7 in the "More Options" box on the CGMiner config page
Obviously that'll mean a lot more sound anyway, but it might not be as annoying.
For a longer term fix you could try buying a spare fan and try swapping fans one by one until you find the problem one.
You could also examine the fans closely with the power off and see if any of them are 'wobbly'
Could it be due to the power board or controller making that noise?