That's deffo a hydraulic tappet with an oil starvation problem, I can understand why everyone has said just throw a set of lifters in and it should sort it (I'd have said exactly the same) but as you've gone down that road and it's now exactly the same you have to find out why the lifters aren't getting the oil they need.
I gather the only time they shut up (for a short time) is just after you've changed the oil?
I'd personally be in a bit of a dilemma whether or not it's worth picking up a known good second hand head (you'll easily get one for £150) and just throwing that on, or you can go down the hope this'll sort it route... ie whip the cams and lifters back out and blast an air line down all the oil ways would be one of the first things I'd have to try.
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