[OniCamaro]

Looking for creative solutions for a pesky exhaust valve rattle problem I'm having. I have the American Thunder NPP axle-back. Sound-wise it is amazing and has all types of improvement over stock NPP that I ever wished for. I bought it used, so don't have warranty.

So it has an annoying valve rattle problem on passenger side only. Now it is nothing like the well-known stock NPP valve rattle issue. This sounds different and not during open/close transitions. More like a buzzing sound. I have already done this solution long back, and it had solved my stock npp rattle problem, so I think clearly actuator can't be the cause. I have also checked this solution out, but I figured it doesn't apply to me as I visually observed that the springs aren't loose.
What I've noticed is that the rattle happens only when there's some potential strong vibration and shaking of the muffler with the valves closed. I believe there are two main situations for that, high rpm exhaust flow and the drone rpm(1.6k-2k) and the rattle happens only during these times(if valves closed). So I'm guessing the noise is caused by the valve edges hitting the walls of the exhaust pipe. Is there anything that I can apply to the tips of the exhaust valve to provide some form of padding to deaden the sound while obviously maintaining proper NPP function and be able to withstand all the heat?
Any other solutions I can try out? I was thinking if it's possible to replace the spring controlling the valve, maybe it's not providing enough torque to rotate valve and force tight shut?
I'm not an experienced car person so sorry if I'm completely off on my take on this and would like help from this knowledgeable community. Thanks!