to turn the 290X's fan down from jet-taking-off to kitchen-stove-fan, so I can hear myself think. This brings hash rate down to 520 or so. And then when I go away
aticonfig --odsc 1000,850
for normal speed of 600-640 MH/s.
Try putting a fan (at least 12cm) right at r290(x) side (near the card fan, 90 degrees to the mainboard), blowing in the card direction. Helped me reducing the noise greatly (it also reduced power consumption, the r290 fan sucks a lot of electricity when turning fast).