In your other thread Jim Anderson said "The vacuum reservoir is located in the right fender behind the wheel. It might have come loose. Remove the plastic fender liner to get access to it." Have you checked that yet?

Anything I say is just a guess at this point. I know the motor mounts can sag in those. I suppose it is possible that has something to do with it. Hard to say without hearing it. A guy today told me one of our power supplies was humming and he thought it had a bad contactor but to me it was squealing so I thought bad capacitor...

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra