iMac PPC G5 cannot find startup disk/HD

Hello. i have a PPC iMac G5 that was donated to non-profit agency. I initially installed Leopard. Shortly after, it began to run the fan for long long periods of time before starting up. I have learned that this usually indicates a problem finding the OS.

This continued for 10 or so startups and then, suddenly it stopped being able to find it at all. I didn't know if the HD had failed or not, so put in a new one just to start over.

After putting in new HD, and starting back up, the computer turned on fine, but then blinked question mark and file folder icon. I then put in the install disk for Leopard. The install disk booted up fine and began the install process but when it came to choosing the disk to load the OS it could not find one.

I have since tried doing the same process with an older TIger install disk also. When I try from the Tiger disk, the computer starts up, shows the Apple logo, and then just runs teh fan incessantly, but never reaches the install screen.

I have read many posts here but can't seem to find the right one. Any ideas? Suggestions?

Thanks so much.

Will