Rough noises/rumbling when load changes

Hello everyone,

Drive a SEAT Alhambra TDI 115hp with a 6-speed manual transmission, model year 2004.
If I am in the 2nd or 3rd By playing with the gas and easily switching between push and pull operation, I can create a rather loud noise that sounds like grinding gears.
The same noise occurs briefly when I accelerate quickly with a lot of gas, make a quick gear change, and the tachometer drops from around 2500 rpm to 1300 rpm when I engage the clutch.
Error memory is empty.
I initially suspected a pan, lid, or something similar, but everything feels solid.
My idea is that the shift cable, or the shift shaft, is what makes the 'clunking' sound when it's being 'manually' engaged.
Maybe one of you has a brilliant idea before I start looking in the wrong direction.

Thank you.

Suther