I have been working on these since they came out 19 years ago and each time they do something to make them quitter you hear something else. Well I have also owned a 13, 15, 18 and now a 20 I know for a fact my 18 made the same noise the bulletin states as a tick but I would agree it is more of a thump and this is more noticeable in my 20. The noise changes as the oil breaks down and usually the noise is gone before I change it. I can’t say for sure what the noise is exactly from but I can say without a doubt it is normal.
The real question is has anybody had or heard of a duramax with this noise fail?