Is it tapping or pinging? Could be valve train, piston slap, or pinging from carbon build up. Did you have the head off? If so did you scrape all the built up carbon?
The 4.2 is noisy at best anyway, check your timing, EGR system, make sure it is getting vacuum. Mine in my '80 sounded like a Diesel due to wear, so naturally I ran 15W40 in it and it was great for another year until I hydro locked it, still managed to drive it home back firing and stalling the whole way, then I swapped in a AMC 360.
I'd say if your timing is good and EGR working properly, and you replaced all the valve train than its most likely piston slap, word is to put skirted pistons in to sort this issue.
Don't put that carbon removal crap in or it will clog your new pump.
If it were me I'd retire the old 4.2 and install a 350 SBC, you can find all the swap bits for cheap used, and the engines are a dime a dozen, and may net you better gas mileage.
'79 CJ7 304,'80 CJ7 360, '81 CJ8 304, '97 Sahara..All Gone
'10 Powerwheels, plastic wheels.
'12 JKU Sahara, 35"Duratracs, Spacer lift, 4.88s, Truetracs F/R. Flat Fenders