Does it change pitch with vehicle speed in a straight line? Does it make the sound stopped and turning the wheels? Is it noticable over bumps?

 

I had something like that in my old FSB that drove me nuts when it started. I gave my receipts to the new owner so I can't go back and look it up but... pretty sure it was a bad front wheel bearing.. my tire shop fixed it.

 

Check your driveshaft u joints, it is common especially if it increases or decreases with vehicle speed.

 

Second this

 

does it sound like this?

if so that's definitely you're driveshaft.

 

Actually drove it a bit more yesterday and noticed it increased with speed/acceleration and thought drive shaft area. That's exactly what it's doing/sounding. But,as with all project vehicles... I stopped by the store on the way home yesterday and it wouldn't start when I came out. The battery/starter wire had broken and the only tools I had with me were a pocket knife and multi-tool. Tried to 'rig it' and get it the other .3mi home,but no luck and it got dark on me. $60 tow bill for a bag of chips. :cry

 

It's probably his double cardan

 

Are aluminum driveshafts decent on these trucks? Or would just replacing the u joints work ok?

 

unless you're planning on racing I wouldn't bother with an aluminum shaft. get your current one rebuilt or buy a new one from bronco graveyard. That's where I got my last one.