If you've had a mechanic look at this then he will have identified detonation if that's what it was.
I would have expected him to know valve gear noise too.
My 190E made an awful racket when warm, but not when cold or really hot. This turned out to be a crack in the exhaust manifold. Maybe your noise is related to the exhaust system somehow.
Sorry I can't help more but descriptions of noises just aren't the same as hearing it!