Strange engine hum during winter - MAF?

[CrazyBoutAFordTruck]

I have a 2000 Ranger, and for the past few winters, it makes a strange hum at times when it's cold outside. Here's a typical rundown of how it happens.

I crank the truck in the morning when it's about 40 degrees or below. I let it warm up for a few minutes before I leave. As soon as I slip it into gear (R, D, doesn't matter), a low-frequency noise begins to hum fromt he engine. It only goes away when I punch the accelerator, or gradually as I accelerate down the road. Sometimes it comes back, and sometimes not. If it happens while I'm driving, the sound will notch up in pitch until -- if I'm stopped at a red light -- it can cause the truck to shutter and die. The sound can also start when the truck is in Park, idling, if the heater is on. In these instances, it happens when the compressor switches on.

I mentioned the MAF sensor because the sound seems to originate in the air flow corridor. I took it out for a spin last weekend in cold weather, and pulled over and popped the hood when the sound started. As I tried to locate the source, I leaned on the air intake tubing that runs from the air filter to the engine, and the pitch changed.

This is only a cold weather phenomenon and mostly just annoying. Has anyone ever come across this in their Ranger? I'd love to fix this once and for all.

 

[Big Jim M]

Crazy most cold weather sounds occur from cold grease.. With that in mind I'd be about changing the two idler bearings on my fan belt. If they have 50K on them or five years they need changing anyway.
Summertime them bearings tend to squeal not hum.
Big JIm

 

[masterbrenden]

or maybe even the ac compressor clutch or bearings
i have seen those make that kind of noise as well
have you tried running the motor with the belt off to see if that is where the noise is coming from
just don't run it for to long

 

[Bird]

Typical symptoms of the IAC valve (Idle Air Control) starting to have problems - especially the hummng. You can take it off, flush with a little carb cleaner then lube with WD-40 to see if it helps, but you'll eventually have to replace it.


Bird

 

[CrazyBoutAFordTruck]

Thanks, everyone, for replying. I'm going to work on it over the weekend and see where it gets me. I suspect the IAC valve may be the culprit.