another guy with a tapping 871

[Hammerhead]

Pop the rocker cover and check your Jake rocker detent strength.
If they don't "snap" into place firmly, but pop and wobble your Jake check valve springs are weak and the check valve and rocker shafts are worn.

YOU WILL NOT HEAR THIS NOISE WHEN THE JAKES ARE ENGAGED! Why? Because the Jake rockers have Pistons in them that are contracted when not operating, creating a significant gap between the exhaust valve pin and the cam lobes. When they are held in place by the check valves, they are sitting there on the shafts motionless not touching anything. Once engaged, they ride the cam lobe and open and close the exhaust valves at precise times.
When the springs get weak, and/or the check valves and rocker shafts get worn, they start to sag a little, and the exhaust valve starts to hit the Jake rocker, knocking it into the cam because the piston is contracted and it has a significant clearance, it moves hard and fast enough it makes a tapping or knocking sound.

This is very common at or around your mileage.