I keep hearing an intermitten humming sound from the engine. It comes and goes. Any idea what is causing this? 2003 F-150 4.6 Litre engine.

 

First thought that comes to mind is possible idler pulley bearing. Sometimes a vacuum leak not big enough to cause serious engine problems can make some crazy sounds also.
It would help if you could tell us if it is coming from the front, which side, or rear of the engine. If you remove the serp belt and spin each pulley the belt drives could possibly help determine if one of them is becoming borderline on failure. You would look for looseness or noise/roughness when turning the pulley.

 

Thanks Hanky I'll take a look at the pulleys. I've heard that maybe air passing over the intake may cause something similar and also the Idle control valve. Do either of these sound plausible to you? And if it's air passing over the intake-what can be done about this?

 

There have been times when the air filter assy was not completely assembled and it would make weird noises.