First thing to check is the fuel shut off actuator. Then look for door lock action. A slow lock could point to the problem. Also check fuel filler door lock and don't forget the trunk lock.
I am a retired Mb Mechanic. One trick I have used with diesels with vacuum leaks is to spray ether around engine compartment while engine is running and listen for responce.
Working through ours too. About 282,000mi. Exhaust, total wheels, brakes, drive shaft bearing, trans and diff seals,, a tie-rod end, axle boots, and some vacuum. One owner. CDT
What is the biiiiiizzzzzzz sound it makes when prestart ?? My 300D make the same notice,, my dosen't shot down at all no vacuum anywhere shit... Think ill try to learn how to test the vacuum pump and then work my way from there,, do you have the name or something om your car model like the color in the cabin ?? My 300D is the same color cognac i think its called?? Like info if you have Hi from Denmark 🇩🇰
