pulled into a shop last night and my 4.6 F150 2001 was humming at idle. I thought maybe it was the ac compressor bc it's done it before. Without popping the hood the mechanic said it's a vacuum leak probably the one going to the breather. Where is the vacuum hose for the breather? I always thought vacuum leak would be a hissing sound not a hum.

 

well typically yes....vacuum leaks can sound different at times.

Maybe pop off the Serpentine belt....and restart , if the noise goes away then it probably is something in the belt driven system. If it doesn't them ya probably will need to investigate deeper

 

Pcv valce? Is there a vacuum kine going to breather?

 

Pcv rattles when I shake it, I don't see a vacuum line going to air filter canister.

 

There is a hose going from the intake after the air cleaner to the driver's side valve cover, that's the PCV inlet. It's not a vacuum line.

Have you popped the belt off yet to test that theory?

 

No haven't taken belt off it runs the water pump. The sound isn't constant . I MAY DRIVE ALL DAY, THEN STOP AND IDLE AND IT MAKES THE SOUND. Normally, I don't hear it so not sure how taking belt off will do anything since it's a random sporadic sound at idle, occasionally.

 

Unless I have two , my pcv is on the passenger side.

 

The PCV VALVE is on the passenger side, the PCV INLET is on the driver's side. Air enters the crankcase through the inlet, engine vacuum pulls it back out through the valve.

Of course the belt runs the water pump. It won't hurt anything if you pull the belt and run it for about a minute.

Maybe you should pop the hood and have this mechanic investigate further? I'm betting it has nothing to do with vacuum.

 

Iac is my best guess at the moment. It hasn't don't it since that night.