Loudly knocking OM606 when cold
 

In the morning when temps are in the 20, teens, single digits my engine has been knocking louder than it ever has. Sounds like marbles in a glass jar. At low RPM, just above idle there is a distinctive knock, as if one of the six injectors are"nailing". Our 82 had this nailing issue and it turned out that an injector needed replacement...

Anyone know what to do to help alleviate this loud unusual knocking? Any type of certain additive, or is it time to replace an injector(s)?

I have been using Stanadyne all season before this last tank.

BTW my mileage for the last two weeks has also severly dropped. Another reason I'm assuming an injector needs attention... But when the engine is warmed up she runs fine, nice and smooth.