2,25 engine with cold knocking noise

My series 3 engine has develped a strange rattling noise when it is cold and I accelerate hard.
When the engine is warm the noise disppears. It sounds very much like a pinking noise, so I have retarded the ignition to 3 degree BTDC to see if that helps but the effect is marginal. I live at altitude [6000ft above sea level]
The engine has been blue printed and the compression raised to 9:1 so it performs very well even at altitude.
I have always used 93 Octane lead free petrol.
So I'm wondering if it is one of the following:
-fuel problem and some preignition when cold
-piston slap [ the engine has done 150 000km since being rebuilt]
-timing chain tensioner "in between" tensioner tooth engagement
-engine needs a decoke
Any one with a similar problem?

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