2001 Subaru Legacy. Hear a clicking noise when going from park to reverse. Someone suggested it might be the parking prawl. How do I confirm what the noise is. If it is the parking prawl  how do I repair it.

it's parking "pawl"

And it's a piece inside the trans.  It doesn't click.  It's a bar of steel that get's pushed forward by the shift linkage internally.  If there was something wrong with it "P" would not hold the car from rolling.

Is it a light clicking sound "inside" the cabin?  Or is it more heavy and thunk from outside the cabin?

If it's a click, inside the car, you really may just be hearing the brake/Park/Ignition key interlock solenoids.  And that would be pretty normal.  They aren't always audible but some are more "clicky" than others.

Recording of the sound may be helpful too.

I doubt there is a problem with the Parking pawl.  But if there is it means transmission disassembly.

did a few cars in that generation have a parking pawl recall?

I had a loose front ball joint that would click when putting it in reverse.