I would think that if the lockring is under torqued to the point that’s there is some movement and subsequently, noise from the cassette/freehub body interface - shouldn’t there be some kind of evidence such as gouging of the freehub body splines?
Can you you tell us about your freehub body splines and their condition? Are they steel, titanium or aluminum? What cassette are you running again?
Also, please clarify about your rear quick release. What type is it, how tight do you tighten it and what kind of rear frame dropouts do you have? A lack of wheel tightness in the frame can result in pinging. Does the pinging sound occur equally when seated and standing or does it worsen when you are standing?