Re: thump/lurch
About the only thing I can think of might be one of the carbs is running extremely lean. If only one carb is running extremely lean the motor can sometimes continue to idle if the other two carbs are adjusted correctly. The one cylinder with the lean condition is basically being dragged by the other two working cylinders. However, every few piston cycles enough fuel will be drawn into the cylinder to provide detonation. When this occurs it may make the motor lurching and skipping along every few seconds as you describe. A fault in the ignition system on one or more cylinders can also make the motor behave erradically.
As for the metallic sound...could be a small piece of metal in the cylinder or behind the piston. This could from a foreign object being sucked into one of the intakes or could be a small piece of piston ring or rod bearing or rod bearing cage.
Removal of one or all the cylinder view ports is a quick and easy way to take a look inside the combustion chamber and a partial view of the cylinder walls, piston rings and skirt.
