It could be because the first knee is taking a bit more of the load, Helen. I doubt it's scar tissue.
However, while I know the theory about why our artificial knees make noises, it often seems that there's no logical explanation. My right knee clicked and clunked for 5 years and then suddenly stopped, but my left knee never made noises. Neither knee feels or felt looser than the other one.
Maybe some extra icing and elevation for the older knee would help, but I'm not giving any assurances there.
As long as it doesn't hurt, I think you can continue to ignore the noise.