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!

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Cheers, Neil