there's no real way to know, looking at the outside of engine, guessing. being its your 1st diesel, and that is a serious knock. 1 change air filter, change fuel filter as well. the fuel filter i get a (small) length of fuel line, and a small funnel, prime filter from inlet, it will help on restart a lot. next i get diesel purge and in a full tank use the whole bottle. can help clear up fuel system. does oil need changing? if so i get an engine-oil flush, and use correctly, on a hot just driven engine, put flush bottle in, and run @idle(&hi-idle) for about 20minutes, change oil/filter. can part way prime oil filter as well.
as far as the knocking, hope van-guy is correct, cause its possible something had come apart, no real way to know looking at the outside of motor, tho,. could be you have a problem, and it could have gone 'silent' doing something you dont want in there.
another way is to bottle feed a whole bottle of the purge straight into pump, and run return line into can.