Hopefully it should settle down after a couple of thousand rounds. This allows everything to bed in and start working properly. It's the same as buying a new car, it runs more smoothly after a few thousand miles.
If the twang is really annoying you get a few tins of cheap pellets and have a mammoth plinking session in your garden, get as many rounds through the gun as you can.
If the twang doesn't disappear after two thousand rounds you may have a problem. Don't despair until then.
Sub Sea Sniper