Have an oldish build, and it's a bit noisier than i'd like.

P180
Asus A8N Premium
Athlon x2 4200
Scythe Ninja + 120mm Antec fan on low.
Seasonic S12 500
ATI x850

It's quieter than the harddisk, but has a steady hum and fan hiss noise - cant locate by ear where it originates as my hearing sucks. The case seems to resonate a bit too much - applying pressure to the side panel deadens the hum - any tips?

(It's not the graphics, can hear that spinup when gaming)

Hello,

It might be a vibrating hard drive(s), or possibly a fan (or fans) -- or both in combination? Where are your hard drive(s) mounted? Are the fans "soft" mounted?

Unplug all fans to see if its the fans, and start the computer with the HDDs disconnected to see if they are causing it. Because you didnt mention case fans, im assuming you are using the stock antec fans hard-moutend with screws or brackets. If you have the first revision of the case with the thick 38mm fan in the bottom chamber, that is most likely the cause of your problem. It vibrates like hell. After that check the top fan, the mounting can be a bit rattly. A hammer will fix that

The hiss is probaby the stamped fan grilles and the small doors in front of the filters.

That humming is the Tricool fan as said. Especially the one sitting in the lower "windtunnel" . It hums sligly less by mounting the fan with rubber pegs. But the biggest problem is the plastic fan mount. Some Blue tack or play doo, and some foam strips will fix that to 90 % at least. Best way, is to get a Nexus or Noctua fan(s).