To be honest I am not sure what the cause of the Knock could be. Since it happend after a Valve Adjustment you would think that would have had something to do with it; but, it could be somethine else that happend by coincidence.

Is there any chance that it is something like a loose Air Filter Housing banging against something (when mine did this it sounded like something was seriously damaged; almost like a Rod Knock) or the Sheet Metal part that is inside of the top of the Valve Cover (that helps keep Oil out of the Breather tubeing) has come loose?

When the Engine is knocking try rotating the Engine by hand and see if there is a spot during the rotation where it gets hard to turn; if a Valve was Hitting a Piston you would think that when you get to that Cylinder it would be harder to Rotate the Engine than on the other Cylinders.

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84 300D, 82 Volvo 244Gl Diesel