Have a listen. Skip ahead till about a minute, couldn't get it to cooperate right away. It kicks in at about 1:15 till 1:24, then I think twice more. Ive been wanting to record it but rit only happens when its cold out, this morning was borderline but the coldest we've in a while at 25. Video doesn't do it justice, you hear it over the turbo, engine, even the radio at good volume.
Its a 2001 stock except intake/exhaust (as far as I know). Its done it since I bought it
In 2006 under the following conditions:
1. It has to be below freezing, about 26-27 or less
2. Only occurs between about 1550-1900rpm
3. I can make it happen only during very light acceleration, deceleration, or holding steady within that rpm and specific throttle range once it starts. Give it a little more gas or let further off and it goes away. If I had to pin it down, id put it at 10-20% on the pedal. If I hold that spot on thethe pedal it turns off like a switch when I clear 1900rpm.
4. Doesn't matter if the truck is hot or cold. It will do it all day if outside temp stays low. It maybe is easier to cause it if the truck is cold though.
5. Starts off sort of gradually almost like a separated chirp, then builds to a solid whistle.
In the video, the truck warmed up for 10min to get the frost off, then starts about 2 minutes into the drive.
Any idea what this might be? Ill try to get a better video if we ever get some winter temps. Sorry to write a book. Been bugging me for years, finally thought to ask about it.