Computer boots when PSU switched on without me pressing the case power button

[Maloof?]

As the title says, I have a computer that is turning itself on when I turn on the PSU. I don't have to press the power button at all; it simply switches on automatically. At first it wasn't turning on at all; I would unplug the 24-pin and begin to plug it in. About halfway down it would switch on. Now, however, it switches on when I plug in the PSU to a wall socket.

Any ideas? I tried with a different PSU and the same thing happens.

[editline]01:36PM[/editline]

Note that I unplugged all case leads (power switch, etc) and the problem still occurs.

Earlier on the computer would also make cyclic clicking noises, maybe every 1/2 second.