steel and aluminum together will also have galvanic corrosion. add a bit of moisture and you have issues. just dont add anything caustic.....
but wait.....you have green treated lumber in there. that is caustic to aluminum, the chemicals from the treatment will eat the hull of that aluminum...
Knock tests mean nothing
Does it leak with the motor running while tied to the dock, or just underway?
Use a flashlight, mirror and your phine on a selfie stick and look
pull drive (part of annual maintenance anyway)
pull motor (45 minute job)
put motor on engine stand
address starter issue
while motor is out: clean bilge, inspect transom, etc.
drain it, and if you are worried about rust, fill with anti-freeze.
heated storage is one storm away from cold storage. think texas, the chemical plant that went down in winter because the power went out that is still down.
