G'day Anji.
I still think it's the backlash in the gears. It only takes a few thou" to get the tick-tock. turning the motor over the cam is pushing one valve open then as it goes over max lift another valve is on it's way down and exerts backward pressure on thr cam and tries to turn it backwards. Then the cam catches up as it starts to push the next valve.
If you take both covers off (leaving the idler in place) and turn the motor over you will see it happening.
Cheers
Another thought just occurred. Is it possible that the valves could be touching the piston
Yes, iff the valvegiude is not in the correct angle and a little bit out offline the valve can touch the outside from the valvepockets at the inlet valve, because it is bigger diameter than exhaust.
By building up some engines I have learned everything can happen.
Fit new conrods and pistons and by running there was a twittering noise. My first thougt it take a little time to keep oil up to the rockers, they were also new. The noise growing bigger when the engine get worm. I opened the timing side to get the ignition a better set up there was a little alloy flitter swiming in the oil. Damned....another strip down..
It was the pistonshirt that touched the flywheel by a whisker...
Take a negative selection, sort out what not can happen.