The sound is like dull knocking(with a slight metallic reverberation) from under the car when in motion-- usually starts around 50kmh(30mph). Sometimes it is quiet and sometimes it is loud, seemingly at random. I can hear the noise while going straight and turning and with the foot both on and off the accelerator. At 60kmh(37mph) the frequency of the knock is approximately 260 per minute or about 3 times a second.

Thought it was a wheel bearing but got it up on the hoist and everything is tight. Changing from 2wd to 4wd also has no effect. Changed tires, breaks look fine no dragging. Thought maybe CV joint but it doesn't seem to be-- no damage or signs of failure and everything looks clean.
Could it be drive shaft? Does the transfer case or diff or whatever need fluid?

It's a great car old as she is, would love to keep it running.
Any ideas at all?