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OK, we have to disable FW rule generated by wizard (last "confusional" wizard step...?)

even if unchecked, it always allow communication between clients (ping/rdp/etc)

You have a predictable pattern ;)
as you have surely noticed, the increase is only about 5% in 24h and is only appreciable considering it in the average of a discrete range... compare samples at set times will only let you losing your time  ;)
I collected (via SSH script) the total sample of the processes every 60 "for 24h.
Now I know what  8) 
... It's netflow... Disabled and now CPU stay flat.

So: should I keep my eyes on top for 24h ?  ;D

in other words, something is eating CPU time but we don't know what...

the question does not concern the% of the CPU but the curious growth (slowly progressive) and the subsequent sudden re-positioning downwards after a constant period of time...

on almost 24 h cycle, CPU scale Up and, finally, Down and slowly Up again...