I would say the noise is popping between the 2 pistons. Compression test proves this. Normally happens between bores 2&3 on tdi
I haven't followed the story from the start, but, why was the head gasket changed to begin with?, ... is there a lurking root cause to look for?
Has there been some leakage between 1 & 2 also? There look to be some dark marks on the gasket. If so, is the head truly skimmed flat?
As an example, if the milling machine had an adjustable head which wasn't truly at right angles to the bed of the machine, the cutter would cut a slight dish rather than perfectly flat. You can sometimes see this effect in the pattern left by the cutting tool.
Similarly, is the block surface flat? Have the threads in the block pulled up?
Holed radiator originally back in October, over heated but changed rad on the side of the road and seemed ok.
Gasket went over Christmas clouds of steam so skimmed head and changed gasket. 6weeks later and well.... pic above
Not changed a petrol head gasket for years. Just a thought, is copper gasket the norm on petrol engines these days?
They were always trouble on the diesels, I have been using composite for years when I have done them.
I'm wondering if the torque wrench I used is wildly out.
BB how about new head bolts? and did you run a tap down the threads to clean the up, so you got clean holes to work with?
Didn't feel I needed to, bolts spin down happily by hand.
You can check them with a big spring balance, or borrow someone elses for comparison.
They usually last for donkeys if reset to zero after use.