1999 ford expedition 5.4L humming

I have been hearing a loudish (I can hear it in the cab) humming noise coming from the engine bay. I don’t hear it when the truck is cold (1 to 5 minutes after starting), seems to get louder at it its peak (the more I drive). When i open the hood, the humming is coming from the intake. I am a pretty handy guy, just at my witts end right now…not sure, but I was thinking brake booster? HELP!!!

Does the humming occur only when the engine is idling? Or does it occur at all times?


If the noise is only there at an idle, it could be a faulty Idle Air Control valve. Look online for a picture of one, then find it on your engine. Put your finger on it when you heat the noise, if you can “feel” the noise you’ve found the trouble.

I only hear it at idle but I am assuming it drowns out once the engine noise kicks in.

Its not the IAC Valve, I replaced that a month ago