Had a similar problem with a friends machine recently, and even swapped the power supply over, and still nothing. Removed the power lead for about two minutes, replaced the power lead, and with the lid off, checked to see if the cpu fan 'twitched' when the power switch pushed. It twitched, but then noticed a power transistor had blown a leg off itself, and pitting in the transistor where it had burnt out, causing the pong.
I usually find power supplies are easy failures, so try to see if the cpu fan twitches or not when powering up. Have a close look at the motherboard for any signs of overheating. You may be lucky and its just a psu.
cheers