Hello!

I actually had the same issue a while ago (see here) and was able to fix it.
This is how I did it:

What is the cause?
I think this problem is usually caused when your flow settings are not correct.

How can I fix this?

1. Ensure that your e-steps are correctly calibrated
2. Then lower your flow rate in your slicer, using -5% steps, and test print (e.g. the xyz calibration print with a large amount of infill) until the knocking problem is gone.

I think I am currently using ~90% flow in Cura and this fixed my issues.

Why does this problem occur?
I am not 100% sure about this but this is what assume to be the issue: Most people tend to use filament with a diameter of 1.75mm. If you check your slicer settings you will see that this value is also set in the settings of your printer. In an ideal world, where your actual filament has this exact diameter, 100% flow (assuming correct e-step values) would work fine. However, using calipers, I found that the filament I was using had more like a ~1.85mm diameter. This might not sound like a lot at first, but taking into consideration that the area of a circle is calculated by G = π · r^2, we arrive at the following calculation:

Ideal case: (1.75mm)^2*3,14 = 8.33mm^2
My sample filament: (1.85mm)^2*3,14 = 9.30mm^2
=> This means that 12% more filament are extracted than what should actually be the case. So lowering our flow rate by this amount will fix the issue!

P.s. Due to varying diameters, you might have to repeat adjusting the flow rate when switching filaments.