Tapping/knocking sound

[Hammerhead]

You described the sound as "tapping, like marbles" that's why I went to jakes.

If the sound is more solid, like a small ball-pein hammer on steel, has the crank damper been changed? If the viscous fluid in the crank damper hardens, the inner steel weight ring bounces off the outer shell, but only does it under torque load so revving the engine while stopped reveals nothing.
If the damper has never been changed, I suggest you change it. If the viscous fluid has reached the end of its useful life, then the damper is no longer properly absorbing the harmonic vibrations from the crankshaft and these vibrations can and will play havoc with crank position and speed sensors, as well as lead to premature crank and main bearing wear. It's an 871, they haven't been made in nearly 10yrs. If the damper has never been changed, it is beyond its expected service life.