Impact driver issue: Intermittent springy/twang sound
A little late to the party, figured I would share an image I found while researching my own impact driver's issues. 

I'm guessing that your "twang" isn't associated with the trigger mechanism (or you'd probably have figured that out already). The hammer mechanism has to have a spring to restore the hammer back away from the anvil, and after a lot of use I'm sure the spring can get fatigued. I'll post images of my impact driver once I pop it open and get this section exposed. In the meantime, check out this site below:

https://eztoolhub.com/ultimate-guidelines-in-repairing-a-cordless-impact-driver