If it was a connecting rod bearing the engine would knock worse real fast. Rods bearings once going out won't last long at all. A faint knock however maybe just a bit of a colapsed piston skirt. This could also be a ready to blow condition. If the piston skirt falls off the piston could wedged in the cylinder and lock up, taking the block crank and rod with it. Tell me this does the knock go way once the engine is fully warmed up? Is the engine warming up fully? Some motors have alot of clearance between the cylinder and pistin. When cold they will chatter a bit. Once warmed up fully and everything has expanded they're quiet.

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1984 280SEL, 62,000 miles
Euro model in the USA
1983 300D (Totaled out 10/2004)
New Factory Mint Green paint
Palomino int
1982 240D (Steak dinner for two)