With respect, what Lucas said is nonsense. If it's the ping I'm thinking of (there may be more than one source of ping!) it's most likely to be a worn O ring allowing metal to touch metal (always assuming it's coming from the rifle and not the bottle!).
The most likely culprit is the front bolt buffer o ring - check that first, particularly if you're running a QEV. What grease are you using? Also check the bolt guide the circular disc in the fire chamber) for signs of wear.