As I get older and since I'm pretty much retired, I don't move my joints near as much to their limits as I previously did. I do find that they get stiff and pop or feel more like gravel is in them.
Thankfully this has not been in my knees. But they do, as they have most of my life give a loud audible pop when I squat down the first time. I try to move all my joints throughout their full range when I think of it.
I've considered going to a gym as certain resistance exercise will help. However the COVID thing needs to get under control first.
No idea if this might be some of your issue or not.