Diagnostic code: HDD fan or HDD?

I have an older iMac I would like to put back into service. Model is iMac7,1, EMC 2134.

Leading up to its demise was a soft click click.

When I boot to the external backup drive, all is well. No internal drive is found by disk utility. Can still hear the same soft click click.

When I run the DVD diagnostics, long, I get this:

4MOT/1/40000002:HDD-0

My sense of things is that the hard drive is dead.

But all the googling I have done on this exact diagnostic code suggests that the HDD *fan* itself might be bad. I am hesitant to get a new HDD, at least without being able to source for the HDD fan itself.

Questions:

Since I cannot even see the HDD, should I simply replace it?

Since the diagnostic code is not totally clear, how does one obtain the HDD fan?

Is the HDD fan an integral part of a replacement drive?

Input appreciated.