A friend of mine had a tapping noise coming from his engine(Zafira petrol) he tried everything and was about to strip the engine down and he removed the spark plugs and was fiddling about with one and he found that it was loose and the top of it moved up and down ....this was the cause of the tapping noise....i know yours is a diesel but it does all point to the glow plugs . another experience i had was a ticking noise on a Focus i owned,this turned out to be serious.....it was a little end bearing that had gone......i replaced a conrod/bearing and it was ok ....i sold it not long after as it never seemed the same.....good luck....hope its the glow plugs 👍