Diesel knocking please help

[Redfar]

Hello to everyone,
My father has a mercedes w123 200d,and now he is having like a knock when he press the gas expecially on second and third gears.We tought that it was the diesel filters we changed everything,the one with the tank the big filter and the small plastic one but the car stays the same,we checked all fuel lines to see if there is a cracked one but nothing.
Now we are thinking that is something from the injectors or from the fuel pump but the car is not making any strange noises when it's free the engine revs normally.
The problem is most,when the car is loaded when he press the gas it knocks is like it's cutting the fuel.
Can someone help with this pls?

Thanks