Hi John and welcome to the forum, Brian has already covered why it pings and sleeving the hammer tail is a solution but if not done properly can cause problems as the movement of the hammer spring against the sleeve causes small bits of heatshrink to shed and slow the hammer. The only permanent solution is to fit the external power adjuster kit which has a new design hammer, spring and power adjuster head that can be used the adjust the power without removing the stock.
If the rifle is grouping well then i would be tempted to leave it alone but if the ping really gets on your nads then try heatshrink. If you fancy the ext power adjuster then drop me a PM. In fact if you have any questions at all then please don't hesitate to contact me.