rumble hum noise
 

1996 f150 4wd noise starts after 25mph doesn't mater 4wd or 2wd tried in nutral , nothing makes it go away , also checked that 4wd is engaged and that is fine also

This could be a lot of things. Just to be clear...you have to be moving above 25 MPH to get this to happen? And once you slow down below 25 MPH it goes a way??

Have you checked to see if your tires are good, no broken belts or tread separations?

If you jack up each tire in the driveway, can you spin that tire and hear or feel and extra noise ? might be a bad wheel bearing? brake ?? or U-joint?

Does your differential cases ( front and rear), have enough gear lube in them? What about your transmission fluid level?