All of the iMac g3's have screen problems. It was the era of bad capacitors. The iMac g3 models a,b,c, & d had an internal fan. Verify that it is working. The repair of CRT's is a lost art.
The problem could be heat related. You could try increasing the air flow. Try putting a fan below & direct the airflow up.
You could look on ebay for Mac OS 9.x. Make sure you get the full install version . A large gold 9 on the cd & box.
You might want to look for another cheap machine. You would be better off with a newer machine. Look for a mac mini.
I readed other messages and one said it might interfear with the cables,
I would think that this would show up when you started the machine & not after it ran for awhile.
I recall that placing headphone in the head phone jack turns off the speaker sound. I think that a dummy jack turns off speaker sound too.
I knew the fan was working beacuse I put my ear up to the matchine and it was loud and working. And how do I know mine is model a,b,c, or d? Also, when I turn off the screen, it makes a weird sound, like if it were on fire. and so I just put a fan in there up so the airflow would go up? Im still a little confused.
a -- 233 2mg vram
b -- 233 6mg vram
c -- 266
d -- 333
Query by serial number
"A serial number is a unique, identifying number or group of numbers and letters assigned to an individual piece of hardware or software. It's used for various things depending on the product / brand but what is your Mac's serial number for and more importantly... what is it hiding and what can it do for you ?"
( hint by K Shaffer )
I think that you have a capacitor problem. The power off sound is an indicator.
You could try cooling your iMac some more.
How to fix iMac capactor problem.
What are you wanting to accomplish with this machine? Is it a hobby machine?
I'd recommend at least a G4 processor & 1ghz. Most people would say an early intel processor Mac.
That sound is a bad flyback transformer the flyback transformer is what makes the hi voltage for the monitor to work and would make the screen flicker if it is going bad. I just got 5 of the tray loading imacs that have this issue if yours makes a hissing nose when off i am %100 sure it is the flyback transformer
Now, after about 5 months of it being off, i started it up again. It did the same problem, but even faster (about one flicker every 1 second) and now i turned it right off beacuse i was afread that if it went on longer, it would of just gave out. Can you find me a place were i can get a Fly Back Transformer. And it does hiss when it flickers. Can i get a Fly Back Transformer off ebay? were should i get it? how do i replace it?
I think this is the right Flyback You will have to open it up and cheek the #'s on the flyback. This persion from this thread http://forums.macrumors.com/showthread.php?t=1304637 (post # 18) ordered one for his imac and it worked ok! but remember CRT's can hold Power even when off so make sure you know what you are doing before you open it
Ebay Link to flyback transformer http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=290595654328&ssPageName=A DME:X:RTQ:US:1123