High pitched squeal under dash
My 99 Grand Marguis GS has just started emitting a high pitched variable tone squeal somewhere under the dash on the driver side. It only does it when the car is in gear. If I put it in neutral it stops immediately even when coasting. It stops upon hard acceleration and then resumes when I let off the gas. Would it be the computer?
Most likely a vacuum leak, do you have auto release on the park brake? If so it could be the park brake release vacuum control valve, it is part of the Neutral safety switch. You will have to get up under there and check it.