Could it be the exhaust port gasket about to go? Mine always made that sound but quieter, then on a road trip, I restarted the truck at a truck stop and it got louder than that and blew right then. Then it was much louder!
I had come from Chester, IL and was headed for Centerton, AR. Had to drive to centerton, then to Memphis TN and beg them to fix it for me. They did the job, very kind of them to do that for me. I was in no way prepared to do it and had no gasket with me, so I could have stayed where I was and overnighted it and risked breaking a stud or worse, or drive it to Memphis. Try driving a deuce from Springfield, IL to Memphis TN with a blown exhaust gasket, the fumes are unholy bad and it is not for the faint of heart.
Anyway, if the knock is coming from an exhaust gasket leak, you will feel the puffs of hot exhaust. A cheap fast thing to eliminate.
Another is the air compressor, if the lube flow is not enough,
DIAG Tool: Get a bic pen casing or something long and hollow like a 1/4" copper tube and put it on the end of a 3 FT hose and put the other end of the hose to your ear. use this stethoscope to poke around and find the racket. Remember the old "language lab" headsets that didn't have speakers and a cable, but had a hose instead? Those are well-adapted if you can find any.
