115 Mercury Outboard Powerhead Pull and Teardown - Why Does it Knock?

This Mercury 115 HP, 4 cylinder, 2 stroke engine has developed a very audible knock that seems the worst at idle. The engine runs fine otherwise and has been running like this for a while, except that the noise is increasing.
Diagnostic work has been done to rule out as much as possible, so the time has come to remove the powerhead and disassemble it completely to find the cause of the noise.

00:20 What is the noise?
00:30 What has been done and what is the plan?
02:05 Start Powerhead Removal - Disconnect cables, hoses and electrical
03:55 Remove Lower Cowl
04:10 Remove Shift and Throttle
04:36 Remove External Electrical Power and Grounds
05:55 Remove Powerhead Studs and Hoist Powerhead
09:25 Remove Airbox & Oil Tank & Bleed Hoses
13:25 Remove Intake Manifolds and Reed Blocks
15:30 Remove Flywheel and Stator
16:30 View into Crankcase at Bottom of Pistons
17:00 Remove Hoses and Wires
19:30 Remove Starter / Coils / Ignition Wiring
24:10 Remove Exhaust Cover and Inspect through ports
28:18 Remove Cylinder Top Cover
30:00 Remove Oil Pump
31:40 Remove Lower Main Bearing Seal Housing
33:30 Remove Lower Crankcase Housing
37:35 Remove Crankshaft, Rods, Pistons as Assembly
39:00 Inspect Pistons and Cylinders

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