Hello, I've got a problem with my iMac. When I plug the power cable in, the first LED turns on and the PSU is silent. When I try to power on the iMac (press power button for 10secs and release) the PSU starts to make a noise. If I press the power button again (short press, guess it shuts the mac down again) the noise stops. Sometimes the first LED goes off for a brief moment and comes back on again. Is it the power supply or is it the logic board? Did somebody had a similar problem? Please help me. Thanks in advance!