2004 Exp. 4.6 4X4 175K miles

I have a grinding, rattling noise on acceleration. Only when it's warm and only between about 1500 and 2200 RPM. I thought it was pinging and had the plugs and boots changed. There was no CEL and it showed no fault codes.

It still makes the sound but I realized it only in happens in 2WD. When switched into AWD it doesn't make the noise. Switch back to 2WD and the noise comes back. I did it 15-20 times just to make sure. Again, only when accelerating and only when it's fully warmed up. Where do I start?

 

Check the u-joints for the front driveshaft to see if they're loose.

 

Loose front U-joints? How could that happen when the noise only occurs while in 2WD, not AWD?

 

See my videos please!!!!!!!!!

 

When you're in 2WD the front driveshaft has no load, so it 'floats' and has slack. When you switch into 4WD that slack goes away and takes the noise with it.

 

I had a similar issue with my '03 5.4 and thought it was the rear. Same symptoms you are describing. It turned out to be coming from the exhaust system. Some sort of resonation from inside, possibly due to a broken baffle or something. I found this by power braking the truck up to that rpm range to see if it would make the noise without the drivetrain turning and it did. It would only happen when the truck was good and hot and the exhaust had time to heat up from driving a bit. I know another guy in my area with the same exact thing happening and his was exhaust related as well.

 

Makuloco is on the track.
You have an IWE that is not fully engaging or is worn to the point it is sliping.

 

I may have the same issue. Mine does it if I am pulling out on a hill and turning at the same time. Makes the sound very similar to flywheel grating when you have a broekn motor mount

 

Hmmm. After reading these "bad muffler" comments I would be happy to know this is my exhaust (and not my rear dif). Same/similar scenario for me.
I first noticed this odd sound on a hot day after running 75MPH for 40+ miles, I came to a stop and as I pulled-away (accelerating hard, turning left and slightly up-hill) I heard a very strange howl/honk-like sound for a brief second or 2. The sound was weird because it made a sound "bigger" than what I would expect my truck could generate...I figured it was noise from a semi behind me because I only "heard" the noise but did not "feel it"... but then it happened later that night on another left turn. Disappointed, I then generated my very own loud & growling howl followed with some colorful words. I will check back/update here if I can confirm my 2004.5 FX4 5.4 has a bad baffle in the 3 year-old OEM muffler or if the 12 year-old rear-end is funky (it's very intermittent).