When I was shooting a 3D Rover, I put a small piece of moleskin over the elevation adjustment screw to cushion it. I didn't like the metal to metal noise it was making. Naturally I had to reset the elevation after that, but I carefully measured the height of the prongs before putting the moleskin on, so it was no problem.

Don't know about the twang soud though. Check for fletching contact. I assume you've got teflon sleeves on the prongs. If not, you need 'em. Then try wrapping the base of the prongs with moleskin. Might also tighten up the return spring tension a little and see if that helps.