I have recently noticed a growl sound when driving > 40 mph. The noise seems to get louder the faster you go. Sound is constant regardless of load, even makes the sound when dropped into neutral.

I replaced the wheel hubs, and also the front axels and the noise is still there! I also noticed the front drive shaft spins when in 2x4. Is this normal?

I have also checked and replaced fluid in the front diff. and also had the tires rotated and re-balanced...

Can you guys help me out?

Thanks, I appreciate any help.

My truck is a 2004 F150 5.5 4x4

 

Could be the CATs. My truck made a growl and it was the driver's side CAT. They are problematic on these trucks. Good Luck