My last option is to take to the Apple Store! Can someone help before taken a trip to Apple.
Apple won't work on a 2008 iMac. They may not even look at it. See their service policies here:
Obtaining service for your Apple product after an expired warranty - Apple Support
Did all their diagnostics procedures over & over again!
Please pardon us if we repeat what was previously suggested; we don't know what the tech told you to try.
The computer is making the right sounds for having a working power supply. They are a common cause of death in older Macs, but yours seems ot be OK.
Next on the suspect list is the hard drive. Did the tech recommend a Safe Mode boot? Start up your Mac in safe mode - Apple Support
Do you have the grey system install/restore disks that came with the computer? If the computer completes startup when booting from the /DVD (hold the c key sdurung start-up) that suggests that the internal drive is kaput.
Has the computer been off wall power for an extended time, like months or longer? You can also try starting while holding the OPTION key. That opens the Startup Manager that shows any bootable volumes that the computer can access. If your hard drive shows up, select it and continue starting. If the computer has been off wall power for a long time, the internal backup battery (the so-called "PRAM" battery) will die and the computer forgets where its startup volume is located.