Consulting one of the many Model A books I have, it sounded like it could be something to do with the piston rods. Hopefully it meant removing a shim from one of the rods. I had a feeling it might be more than this, so I decided to borrow my friend's engine lift and remove the engine for a proper inspection. We did however determine that the metallic sound appeared to be coming from piston #3.
I am on vacation this week, so I have time to finally look at this. Today, my friend Tom and I pulled the engine out and inspected the underside. Turns out that piston #3's babbitt is deteriorating. At this point, we should be able to fix this problem. My friend Tom should have some spare pistons with good connecting rods which we can swap out this one with a good one. With any luck we should be possible to get the old girl back on the road in time for this weekend's tour