1994 Mercury Marquis loud humming noise

My 1994 grand marquis is making a very loud humming noise coming from under the hood. It sounds like a air plane or rumble strips on the side of the highway. It occurs after my car is warmed up. It makes the noise at all speeds and turns. It will even do it when my car is idling. When I tap the gas peddle it will stop making the noise but will continue soon after that. I�ve had it in for this reason three times and they found nothing