There are times when the lag bolts into the engine beds on these boats have come loose. The wood inside the engine beds has rotted away and the bolts don't feels loose because they are rusted to the bottom half of the shock mounts. When you reach a high enough load on the engine the one good mount will keep it from lifting too much, but the bad mount will result in a sporatic banging that sounds very much like someone smacking the hull with a hammer. Check the mounts with a crowbar, but don't play HULK with them, just test them within reason. Ripping out a fairly firm one WILL prove they need work, but will not prove that was your banging.