For new main engines build after 1998, the noise of exh leak comes when you did not put properly original lagging and metallic sheet cover on the exh bellows just after the cylinder. Normally engineers do not cover it to check for any exh leakage during trying out after decarbonization of a unit. Resulting noise is always leads to suspect of exh gas leak or knocking of the unit.
If you put it properly the sound will disappear. The sound is always worrysome. Hence do not forget to cover properly the exh manifold.