I have a 19hp 42" cut troy build with B&S engine that made a loud pop the other day and now it vibrated you 2 death when on low throttle. It sounded like what a rock would in your mower BUT the PTO was not on. It makes that sound now at least once while in use. Thanks.

It sounds like the engine I have. Check the pistons, mine has a broken rod runs but one piston does not move. It also cracked the crank case behind the starter where it is hard to see from outside the engine.

Remove the exahust and put a piece of flat stock in the valve and slowly hand crank and feel for a little tap if it's there you have to do a full break down of the engine.it is easy aslong as you take your time.

Just remove spark plugs put a screw driver or something in the hole turn real slow by hand , and see if it pushes back if does not push back pull head and inspect.
Luck