1997 F150 Ford Groaning Noise

[Bravada]

There is a very loud groaning or humming noise coming from under the back of the cab area. It's not steady, it "hums/groans" for about 3-5 seconds and then quits and then continues, while in motion forward and reverse (under 5 mph, haven't had it on the road to test at higher speeds) and in Park. Then after about 1 minute, in Park, it quits.

Anyone know what that is?

[justen]

Driveshaft U-joint? I once had a odd funny noise like that but it was the back wheel. I was going to tear it apart one day to see what was up. Here one of the lug nuts was sort of loose. I tightened it. And noise stopped. Only thing I could think of was the noise of the nut slowly turning working itself loose.