I have a computer I've been working on, and the power supply is 20-pin ,and the motherboard is 24-pin. Is it necessary to get an adapter to the 24-pin? or will it run without. I've tested it without and:
-When I plug the power supply into the wall the green light comes on and there's a high pitched noise
-when i press the power button on the front of the PC the light dies immediately, the fans move for a second, the keyboard lights flash once
-everything dies except for the high pitched squeal
When I push the power button again nothing happens until i unplug and plug the power supply back in and the process repeats.

Anyone else have or know this problem?

bad. dont run a 24 pin motherboard with just 20 pins or you will probably fry it

your motherboard is now probably dead

buy a 24 pin psu man, they are cheap and backward compatible with 20 pin boards.

Running a 20 pin connector on a 20+4 motherboard shouldn't "fry" it, it would either make it not start properly or be unstable when it starts.