hello I bought an imac a month ago and at each startup and shutdown the computer was making a kind of clicking/popping sound when the screen was turning on and off. That mysterious sound was also occurring when the imac was going to sleep mode. My computer was plugged on a power bar and I decided to plug it directly onto the wall outlet instead and guess what? ....no more sounds nothing ....so what do you think that was and should I be worried about anything now ???
sounds like you solved your own problem
if it is working fine now and continues to do so , throw away the power bar and get a new one if you need to , else you can keep using the wall socket