I have a 91 Toyota Pickup with the 3.0L V6. It has had a really quiet clicking sound coming from under the hood for a while. However it has slowly been getting louder. It's a constant clicking/knocking sort of noise that is constant, it isn't a harsh noise but it is deffinately audible. Under acceleration it also makes a louder pinging type noise. Does anybody know what this could be and how I could try to fix it?

You could have an exhaust leak. I have on in my GTA now and it has gotten louder too. You could try to locate it with a piece of flexible hose or tubing, and use it like a stetoscope. If it's coming from around the manifold then most certainly it could be an exhaust leak.
Another thing it could be a is valve rocker out of adjustment. That's just what I can think off for now.

-Ozzy