mercury 40EFi - 2005 - exaust probs

[Fred Especial]

My engine is making lots of noise and smoke out enough by the propeller.
Mercury agent told me it was the exhaust pipe stuck.
It is the first time he heard of this case.
Is corrosion by electrolysis?

[boats.net]

Remove lower unit and look up in exhaust with flashlight for obstructions.

[Fred Especial]

To remove the lower unit, the SHAFT ASSY-SHIFT is loose? or have to do something to release it?
Thanks

[Fred Especial]

'm waiting for the impeller to exchange it and then see the exhaust pipe.