I have a 99 Ford Ranger Sport Edition V6 4.0. Well In the summer time when my air conditioner is turned on, my engine begins to make a low pitch humming noise and it is really loud. This only happens when my truck is at an idle state like at a stop light. I have put a K&N cold air intake and that seems to be the source of the sound. I took it in to get looked at and the guys said its because my intake is sucking up nothing but hot air causing the noise. This can't be the problem because it did this with my stock air filter as well....So no one seems to be able to help me and i want it fixed because it is really loud and annoying. Anybody know what this is?

 

Only when the AC is on? Only at idle?

I had a cracked throttle body on my truck and it made it whisle, but not at idle, only when I was on the gas.

Gotta be a problem with your compressor.

 

Make sure all your vaccum lines and ect. are hooked up. They will make all kinds od noise if not hooked up or leaking. Good luck.

 

I'd bet it was the IAC valve.
Sound like a boat horn or a fog horn?

Try searching for IAC valve here on R-F and see if that sounds like the problems your having. The IAC is not real expensive, but it's not just real cheap either. (I think around $60) I replaced mine on my '99 4L a year ago because it was making that loud humming sound. The hotter the weather, the louder it seemed.

 

I agree with Chris.

I had a 94 t-bird that did the same thing. At idle (especially when cold) it had a howling noise.

I cleaned the IAC twice and it only got slightly better. I replaced it and it was fixed

Once you get it replaced disconnect the battery and turn on the lights & ignition. Wait 5 minutes, turn them off and then reconnect the battery. Then put it through the idle learning sequence. (search here at r-f)

Rich

 

SeminaryRanger Your are a genious!!!!I do believe that is the problem!!!!!Thanks

 

You can rule out the IAC by disconnecting its electrical connector while it's doing the noise.