Start from scratch. Pull every PSU connection to everything and make sure they're properly reseated, as in listen for the click and pull up against it to verify the click. Test again. Essentially, rebuild the box from scratch (no need to remove the HSF though, saves many steps).
If that doesn't produce a change in results, go to a minimal build with no OC. CPU, 1 stick of RAM, and no GFX card. If it POSTs to the monitor, listen for the hiss. Then add, in order, with a test each time:
OS HD (if you have a way of cloning it before this test, do so)
Any other cards yanked in step Zero.
You don't say if you OC, but if you do, don't try to reestablish until the box runs 100% at stock speeds.
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