Whistling noise
 

I have an 04 F150 STX 2wd, 4.6L, auto, 77,000 miles. During acceleration I'm hearing a whistling noise coming from under the hood. It starts out as a low pitch whistle that gets louder and higher pitch approaching each shift point, and remains steady during cruise between 50 and 70 mph. It goes away if I let off the accelerator, but comes right back as soon as the rpms start increasing. It sounds like an air leak, but it doesn't seem to affect performance or operation of any other systems. The engine compartment is clean and all components and hoses seem to be in good condition. There are no mods, so everything is factory stock under the hood.

I have something similar (04, FX4, 5.4, auto, 4x4, 80k miles). Shows up at 49/50 mph and is gone again by 54mph. Also goes away if I let of the gas. From what I have seen this seems normal and is nothing to worry about.

Check the connections around the air filter box, and air tube to throttle body for loose clamps or cracks. It might be air entering around the air intake assembly.

This is the same noise we have been talking about in this thread:

https://www.f150forum.com/f4/weird-noise-3783/

Whistling noise solution
 

Thanks for the feedback. I've checked all the air and vacuum connections and they're all good. However, I came accross another thread on this problem and the whistling noise was found to be coming from the alternator. Apparently, the rear bearing was the culprit. The owner had the Ford mechanic ride with him and he knew what it was as soon as it started whistling. The alternator was replaced without further incident.

I'll check my alternator, but I'm skeptical. A bad bearing doesn't whistle - it squeals. My problem sounds like air being forced through a small opening. It's pitch and volume change gradually with RPM. I sounds kind of like air being released from a boiling tea pot, and it's more noticeable during cold weather. I thought it could be a sticky pressure relief valve from either the crankcase or transmission. Pressure builds gradually in both of these components and the relief valves are designed to open at a certain pressure. If the valves stick, greater pressure will be required to open them and if they only open partially, the escaping air would whistle.

Any thoughts?

I think my 04 5.4L makes the same noise and I have no idea what it is. I notice it only happens when its colder outside. Maybe 35 degrees or lower.

I have the same problem with the whistling noise in my stx.

same problem here 2007 stx 4x4

My solution is just turning the radio up, but would like to know what the problem is for sure.

whistling noise at cruise speeds 2004 Ford F150 with 5.4L
 

Mine seems to be a cold weather problem near the 2000 rpm speed and/or at cruise 62-64 mph. The belt tension pulleys seem to be okay and all vacuum hoses seem to be connected and in good condition. My F150, 4X4 is 2004, 5.4L with auto with just over 70k miles (mostly highway miles.) The temperature range with the problem has been 14 above and down to -15 degrees. The problem just stated this year. I had new tires, brakes and a new die-hard battery installed in 2009, two summers and one winter ago. The noise started just two weeks ago. I have visited a technical college and the instructor test drove the F150, he owns one also, and found the noise during acceleration, near 2000 rpm, and with cruise set. The sound would go away by backing off on the pedal, but return when speed is resumed. No noise in the shop only seems to be there under load.
A parts store mechanic dealer said that he suggests that the serpentine belt be replaced along with the belt tension pulleys. I am still using the original belt and pulleys.
Another person suggested the rear bearing of the alternator.
The only thing that I did so far was to added a can of Sea Foam to the fuel. I don’t really have any good idea of what to do. I am open to recommendations.