iMac family is M4984. Not sure of CPU, ram, etc.

When I turn the power on, it chimes, I hear what sounds like the monitor powering on, but the power-light stays orange... indefinitely. I hear what I think is the hard-drive going "whirr-click, whirr-click, whirr-click" - usually up to 10 or so times.

Tried attaching an external monitor (Apple Multiple Scan 15AV) to it, but nothing shows up on the screen when booting.

Have also tried resetting the CUDA (took out battery, hit cuda switch, put battery back in)

Also, cannot poweroff by pressing the power button. Has to be unplugged.

Any ideas what could be causing this?

A load of iMacs lately have been having this seems 3-4
year old machines, 1st -3rd generation, in most cases it
is the P/A/V Board (Power/Analog/Video) in some rarer
cases it has been the logic board or the convertor.