Why does a 1997 Aerostar 4 liter engine hum at idle sometimes until you hit the gas?
I have a 97 aerostar. A small piece of debris slipped past my air filter and lodged in the Air Bypass Valve so it wouldn't close all the way. I hissed and whistled at idle and when the valve open it stopped. It only takes a few minutes to remove it. Look on the accuator slide and make sure it moves freely. I also have a 97 aerostar, and I also have the humming problem that was described by the questioner. It does indeed stop humming once you press the gas pedal. I think the problem is caused by a bad vacuum booster behind the brake master cylinder. It is not a sound made by the engine. The booster may have a pierced membrane, and it makes a sound like you have pushed on the horn (but quieter). This is especially noticeable when you are stopped and the engine is idling. Revving the engine increases the vacuum in the booster, and stops the humming/farting sound! The brakes on mine still seem to work OK, but I guess to fix it, you need to replace the brake booster! Probably over $200 (isn't eveything!). However, if you lose your power brakes, it could mean you have to really floor the brake pedal to stop that aerostar!