Marauder depinger

The ping is completely internal. It is a sound wave that travels through the tank from the valve and reverberates. Nothing you attach to the outside is going to interrupt it. A set of earplugs is the only thing external to your rifle that would make a difference. 
The factory rubber ring “depinger “ in the marauder is worthless. It’s just taking up airspace in your tube. 
You have options.... Make a bottle brush depinger yourself which is cheap and works fine, or buy one of Tim Hills marauder depingers. I’m not aware of anyone else making them at this time. Tim’s are fantastic though. There’s also the pvc tube depinger that you could do, but keep in mind the more material you stuff in the tube the less shots per fill you’ll get due to the reduced air capacity. I’ve done the bottle brush and have bought a couple of Tim’s from hillairgun. I much prefer Tim’s. 
 
Replace your factory steel hammer striker with Hill's peek striker and the ping won't be generated in the first place. Look it up.

Edit: Take two steel hammers and hit them together face to face, just lightly, both are hardened steel and you will hear a ping. Now replace one of the hammers with a plastic faced hammer and do it again. That's the difference.