81 is a non-turbo with the early vacuum pump, right?? If so, a failed vacuum diaphragm could be causing oil to be sucked into the air filter. It would account for the oil loss and if the vacuum pump has other problems, it could account for the metallic sound. If it is the early pump, it has a plastic line going from the pump to the top of the oil filter. Check for evidence of oil inside. One forum member couldn't see evidence of oil when his failed but once he started the engine, he could see oil being pushed thru the hose.
I don't see any evidence that the engine lost oil pressure. There were 3 quarts in there and dropping oil pressure is normal as the car comes to idle.
Have you tried to start the engine again to locate the metallic sound?

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1984 307d 126k--sold 8/03
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1984 300SD 315k--daughter's car
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