Hi to all!

I have a clicking, ticking, tapping, clacking etc noise, at the aft/stern of the boat when the engine is idling or below ~2000rpms.

The voice is like a large plywood plate knocks to another. 

I checked the aft cabin floor, plywood covers, engine room... I couldnt find anything wrong.
I checked also the port side, fridge room, and the cables, hoses...
Remote cables for gearbox and engine are under the main water tank. I dissambled the tank, covered the remote cables, did all hoses with foam. Almost nothing changed.

On seeing/understanding that, some sections of the boat is double layer/wall, I cut the upper layer at 4-5 place, put there polyester and closed the cutten parts back... Nothing changed..

The starboard side, there is a water tank, I have applied PU foam between the wall on the tank, at starboard side.

I cut the closed box at the aft. Checked there. Nothing changed.

I open the aft/stern porches, there is a terrible noise, but because of the echo, I can not find it.
It is not remote cables. Not rudder shaft or system.

When the engine is idling, and there is noise, I go to the pontoon, and I -think- hear the voice coming from a little starboard side of the propeller.

Has anybody faced to a noise like this?

Please help me, it makes me mad, I can not find it!..