Earmuffs would work. Model #s would be better. Anything else seem abnormal? Oil level reads at the full mark? Has the mower been run regularly and the problem just started?

It's a Craftsman rotary lawn mower model #917.378980. The iol level is slightly above the full mark and the last time I used the mower was a month ago. I'm using it to mulch the leaves when it started making the noise every time I start the mower. thanks

I take that the mower is starting anyway- just backfires out the exhaust? If I am incorrect, then let me know. If you think back, the fuel is likely a couple of months old. Ideally, it should be a month old or less. Couple this with the fact that yours is an OHV engine then: It is entirely likely as it also has set for a month that a) the fuel is likely starting to get old and b) the start of sticky deposits on the valves is likely. I would do the following: Change the fuel to fresh fuel. Use an additive called Seafoam in the the fuel and oil to help remove any old fuel deposits. Most any auto store has it. Remove the air filter cover (2 screws) and inspect for being soaked or excessively dirty and change if so. Hopefully after running the problem will go away once this is addressed. Let us know how you do.

It is a backfire; clean(Or replace) the spark plug,and see if it stops.Don't worry AT ALL if it doesn't,because there's no harm whatsoever taking place when it backfires;it's just leftover fuel from the previous power stroke that failed to fully burn,so as it builds up in the muffler,the next power stroke happens,and when the exaust valve opens,and lets out the hot gases,some of wich are still burning,it comes in contact with the unburnt gases in the muffler,and finishes the burning with a little "pop"