I have a '90 Grand Marquis 5.0 EFI with 125,000 miles on it. The only
major problem I have had so far was replacing the transmission.
Starting today, when I have the A/C running or the Defroster on
(compressor running) all is fine and silent but if I have my lights
on, headlights or running lights, I get a loud humming sound coming
from the compressor. As soon as I turn the lights out the noise goes