I've never heard this sound before.

I picked up a 1997 toyota 4runner limited 4WD automatic trany 200k miles on her runs great.
So yesterday pulling out of my driveway I started hearing what sounded like a loose bolt that had a few washers spinning on it. To clearify a 1/2" or larger bolt /shaft with a few large washers on it spinning loosely. So I stop took a look and found nothing that would be making this kind of sound. So i got back in and started off again, this time No noise. The after a few stops and a few miles it started again. After stopping then starting to move again the noise was there breifly then stopped after picking up speed to about 25-30 mph.
My guess is it may be one of my U-joint. Am I guess correct or words of advice for me.

where was the noise coming from ?