Well, the standard method for determining what's causing what's happening (no matter what ``what'' is) is to test each possibility one-by-one. In this case, I'd say that, since you can't disconnect all the fans (the power supply one will always be on), you could disconnect each fan one-by-one. But the CPU will heat up very fast, so power it off and let it cool down, disconnect the fan and power it back up. Listen for a few seconds and turn it back off. You shouldn't even make it past the BIOS.