We have an A1 1.6 TDI with 110k miles, it was running very rough and was low on power, after a VCDS test we decided to change two of the injectors, on first start-up the engine was sounding great, quiet and smooth, but after a 15 miles drive the noise from the diesel knock from the engine is really loud. We have always driven 2 or 3 litre Audis and B** 320 which have been quiet,

Is this 1.6 CAYC diesel just a noisy engine by design? 

Wouldn’t describe them as a crude engine but a1/a3 will always be louder then there bigger a4/a6 big brothers as less insulation and sound deadening due to space and budget. Guessing the replacement  injectors were coded otherwise they won’t work at there optimum with the engine but would still work at a basic level. Check all breather pipes are in good condition as this makes a difference 

Sid2020, thank you for replying, yes the injectors were coded both in the standard 1234 order and after it started to run rough again we recoded using the firing order, there appears to be a lot of confusion over what order to code them, even the main people at VCDS seem to be unsure about what order to code them.

Do you know what year Audi upgraded the injectors, if they did.

I’m not sure about a particular year but the letter after the part number is a reference to what version you have, the higher the letter the newer the version so A would be first version on that engine and B would be a revised version and it just keeps going like that. When coding injectors the diagnostic tool usually just says 1 to 4 or higher if more cyclinders and you enter the individual code on cyclinder 1 injector into box 1 and so on, never seen a tool wanting them entered in firing order so may be worth redoing them, the codes can be hard to read so just double check you get them exactly correct.