That sound is just the hammer bouncing on the valve.
Here's what worked for me. Stronger valve spring, lighter hammer spring, lighter hammer (I have an ACP delrin sleeved hammer). You can actually tell how much bouncing is occurring when you dry fire and listen. Then sweetspot the reg/valve. Higher pressures can help reduce bounce because the valve will close faster.
Everybody: Let's ease up on the red loctite, 'kay? Thanks. MCB Feedback