Small Back Fires ??
 

Over the last couple of days, my truck has started to do a couple of small back fires when I first turn it on (not loud at all it even mixes in with the exhaust noise). If I let it sit to cool down and start it again it will do it. It does not do it while driving on at any other time. What could be causing this? Thanks. it’s a Stock 305 engine. No power adders.