I have suddenly developped a knocking noise from the engine.
it is coming from one of the 8 pullies on the serpentine belt.

If I pull the tensionner back, and the belt stops, the noise also stops.
As I slowly apply tension back, I can get everything to spin, but without full tension, and there is stillno noise. It is only with full tension that the knocking noise starts.

I have removed the 3 idler wheels, and the tensionner, and replaced each bearing. They are not the source of the noise.

Everything spins freely by hand.

The AC works, and no change of noise when it goes on and off
the PS seems fine, no change of noise when steering
the Alt also seems fine, no change of noise when I load it up with everything

THe last thing is the harmonic balancer, (the crank pulley) Seems visually OK

It is really hard to pinpoint with everything so hidden.
If anyone has an idea, let me know.

 

I would try listening with a mechanic's stethoscope. That might localize the sound enough to figure it out.