Can you isolate it down to one cylinder by cracking the fuel lines? just unscrew the steel line from the injector while running - the cylinder will stop firing. if its the nailing cylinder the nailing will go away.

When you isolate it down to one cylinder, swap the injector with another cylinder. if the nailing follows the injector to the other cylinder its the injector. if not, its something else. maybe the prechamber etc.

thats the next step.

it would also be good to have your injectors professionally pop tested and spray pattern checked. its cheap or free at a bosch service shop (if you have one nearby). just cleaning them doesnt mean they are working right.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car