New push mower makes popping sound?

Bought a new push mower Briggs and Stratton 4.5hp 20" side disharge. Put oil in and checked to level. Poured gas in and started it up ran good sitting, and real quiet. Mowed a little gr with no sticks or obstacles and mower is making a little popping sound every couple seconds like a putt-putt sound. Its not like a car backfiring or loud but noticeable on the very first yard mowed. Thought maybe someone has had this happen before. I was 100% my gasoline wasnt mixed with anything, or the gas can has ever been used for anything else, but I don't know what else it could be. Thanks!