I'd almost think, with the ping changing based on weight, that it could be spring twang you are hearing. Never heard of the ping changing based on the pellet weight. True ping comes from the air rushing thru the cylinder.
To me this Hill design works pretty well and doesn't take up much room at all. You could actually 3D print one if you had a printer.
https://hillairgun.com/product/hill-airgun-pcp-depingers/
Could be. I put a heavy spring in when I did the valve work.