My truck has been doing this for the past couple of mornings. Only when its cold out. I start it and when i put it in reverse a loud humming noise starts. Sounds like a horn blowing. Only lasts for about 3 seconds and then it stops. Sounds like its coming from the engine compartment somewhere. I am not really worried about it that much but i just wanted to know what you guys thought. Thanks!

2001 F150 4.6l
13,000 miles on new motor
110,000 on the rest of the truck.

 

If it sounds like a foghorn, and it stops if you blip the throttle, it is probably your Idle Air Control (IAC). Search foghorn or IAC for more info...

 

I agree with ammo, it happens about this time of year. Take it off and clean it. Be careful not to loose bolts or gasket. It is a bit tight back there but even I did it. Clean with t-body cleaner or similar. While you're at it, give the t-body a clean and the aluminum elbow too. Use a toothbrush because it will be gummy. It'll run sweet or if not right away, soon after and you'll be good till the next year or so. I myself got tired of it happening so I replaced the IAC, but if $$$ have to be saved, Christmas or whatever, it's much cheaper to buy time by simply cleaning it for now. Keep in mind it will have to be replaced one day.

 

Yea i already cleaned it once due to idle problems. Guess i should just buy a new one. Thanks!!