Alright, keep your hair on.
........ Maybe no one knows that rifle.
Basic advice would be to strip it, clean up the innards and, put a light film of moly on the spring and round the piston seal.
To reduce twang, you may need a better (slightly tighter) spring guide or, a piston sleeve (made from ally or plastic sheet.
Either will reduce space around the spring and help to dampen the resonance (twang).
If you're going to take it apart, you may as well do both.
.......... There's some good videos on YouTube about making a sleeve.