Problem Solved!
I had the chain removed, thoroughly cleaned and lubed and the sound is gone.
I guess you can't have a descent chain cleaning unless you remove it...
Next time I have a problem I am going for the obvious solution and not thinking the worse possible...
Thank you all guys for your help and advice
ps: I want a bike that can beat
this! It was waiting around the corner and then just vanished in the forest, accelerating ... vertically!