Some old cast iron Briggs had a camshaft that rides on an "axle" that passes through the cam,instead of on its own journals like most others..sometimes those got some slop and so can the gear teeth and create a slight knock or ticking ,kinda a rattle noise..to some degree,I'd call that "normal" for these engines...
could be a wrist pin getting loose maybe too,or a ring thats broken or loose in the groove in the piston,usually it'll smoke too though if thats the case..
..alot of other things can make noises--valve springs,loose guides or worn stems,connecting rod journal loose on the crankpin,even main bearing wear,pretty uncommon to see that on a 16 HP cast iron Briggs though..I had one with a sloppy points push rod that made an annoying noise,until I reamed the block and bushed it and put a new pin in it..ran much smoother after that too..
It smokes at low rpm as well.
It looks like I will be rebuilding the engine too.
The model number is 326431, so if you know where a good place to order a rebuild kit please post it.
I will take the engine out and check to see if someone has been into it before.