Sounds under the hood
 

I have a 96 cougar, 4.6 litre V-8, I just replaced the motor with a motor out of a 04 marquis only 70K on it. Motor runs good, however there is one thing that I have been trying to figure out: after driving the car for a little while, from time to time there is a low "moaning" noise that comes from underneath the hood, it sounds like it may be coming from the air filter assembly, this does not affect the performance of the car and if I give it a little gas it usually stops, I just wondered if anyone else had encountered this and what was their solution. Also, the car was doing this BEFORE I switched the motors, that tells me that it is not something in the motor. Thanks for any suggestions.

maybe its low power steering fluid in the power steering resivour. have you checked the fluid in the resivour? maybe the power steering pump is going bad and as you rev up the engine, the pump turns faster and the moaning stops. or maybe your car has a bad air pump like my mustang had. if that wasnt hooked up it would sound kinda like a moaning. but i would check the power steering pump and fluid first. i am also assuming that you did not replace the power steering pump when you changed the motor.

Thanks for the reply socool, but the power steering pump is good to go, like I said I have heard the sound once or twice while the car was sitting at idle, it sounds like it is coming from the breather tube or the air filter assembly or something like that.