Any owners of 300D's, does your car make any noise after you turn off the key?

My (new and first) 84 300TD makes a buzzing/whistling noise from the EGR valve area (by the hydraulic tank) after the car is stopped. I can feel the EGR valve vibrating. The vibration stops after a couple of minutes.

I replaced all the vacuum connectors in the area. No change.

It seems to me like this is where the vacuum on the engine side of the check valves leaks out after the engine is turned off. If that is true, this noise could be normal. But I don't have any other 300's to compare mine to.

I don't have any performance issues (smoking, stalling, etc) as far as I can tell.

thanks in advance

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84 300TD - 235k - Farbe "Surfblau" bought at 213k
87 300SDL - 131k - Farbe "PimpGold" bought at 115k
00 Klepper Faltboot Expedition Double