Let's play "what part is failing?"

[Undertoad]

Very worst case: the noise is your CPU fan, and it's making noise because the heatsink is uncoupling from the CPU. When the sink is in place the system boots and acts normally. When it's slightly off, the CPU heats up so rapidly that it reaches 80 degrees C in 2 seconds, at which point the mainboard CPU temperature sensor tells the BIOS to shut the whole system down to preserve it. When the sink eventually fails with the system running, it causes a fire which spreads to the papers you've stacked behind the system, and your whole place burns down.

It's probably not the problem, but it *could* be!