There are several
threads about this issue. I'm not stating this will fix your truck, but it did mine.......My truck started this so I done a search on this site and found it to be a common problem.
Issues with the Degas cap and the Whistle noise.
My truck started making this noise and puking around the cap. It's a moaning tea kettle whistle sound. It mainly done it if I was on the throttle hard and when I backed out of it. Pulling hillls and anything under a load made it worse and louder. I tried several things to fix it...... After searching around on this site, I called my ford dealer and purchased the updated cap, part number 9C3Z-8101-B
they looked and said the cap was just released and was the newest version), it was $12. Looks identical except it does not have the sticker on the top.
I've always ran my fluid at the max line, I drained it till it reached the min line and installed the new cap. My new cap took alot more torque as well before it clicked.
I've ran the new cap for a week now. So far the nosie is gone and no more puking. I've only romped down on it 3-4 times since the install, but so far everything is good.