[urq] What causes knock ?

[Louis-Alain Richard]

My car developed some strange sounds, and one is sounding a lot like
knock. It happens only under boost, and it is sporadic. But it scares
me, specially after what happened to Dennis...

The second strange sound is identified, it is the fuel tank collapsing
after a few minutes on the highway, a big "bong" sound. Today, as the
fuel tank was nearly empty, I stopped and noticed that the fuel cap made
the "iiisssssss" sound of air entering the fuel tank. So there are no
fuel vapors going back to the tank, IMO.

Now, are these 2 problems related ? Can a fuel tank that is under vacuum
stops the fuel flow to the pump and starve the engine ?

Then, how can I find the problem with the fuel vapor circuitry ?

And my title question remains: what causes knock ?

Louis-Alain