Can anyone explain this?

1984 300SD I had the fuel pump rebuilt because performance was degrading. Immediately, the car had a squeal accompanied by a slight loss of power at cruising equilibrium (cruising without acceleration or deceleration).
I moved out of state a week later and could not take the car back to that shop. I lived with the problem for months. I blamed this problem on a sloppy rebuild job. Now I'm not so sure.
I just had to replace the ignition switch mechanism behind the key tumbler because it locked up. This mechanism has a vacuum line to it. The fuel pump squeal problem suddenly disappeared. I could park the car for a week and I still had vacuum in the door locks. There was no leak in that system. Perhaps the vacuum to the ignition switch is isolated from door locks. This is a California car. There is some smog system that I believe uses vacuum to modify transmission shift points and the fuel pump is involved. There is a small black box on the side of the fuel pump that I don't think is on a 49-states model. I'm happy to get this sudden performance increase. I just can't understand why changing the key switch solved this issue. Can anyone explain this mystery?
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The engine is shut down by a small vacuum powered bellows that's located in the aft part of the injection pump. The bellows pulls the fuel rack to OFF when the "ignition" key is turned to OFF. It could be that the key's vacuum component was leaking and so pulling the shut down bellows in the injection pump. The noise ???
Incidently, the shut down bellows wears out after about 15 years' use and the motor won't stop when the key is turned to OFF. So when that happens you'll know the cause.
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