Hi I have a 1988 Chevy G20 and after running it for a while it will start making this ticking noise when it's in drive or in park. Took this little video and posted it to YouTube so you can listen for yourself.

 

sounds like you're running on 7 cylinders

 

The frequency of the ticking sounds like it could be valve related.
Sometimes we can get a better idea of where it is originating by using a piece of hose held close to one ear and CAREFULLY pointing the other end of the hose to where we feel the sound is loudest.Just say away from any moving parts and high voltage points.

 

A leaking manifold gasket can also sounds like ticking valves. Check for leaks.
If it is the case, have a look here for clamps: : https://www.kralautoparts.com/

Good luck

 

I bet you take the doghouse off you find a bolt or two on the exhaust manifolds have broken.

 

The most common reasons a Chevrolet engine makes a ticking noise are low oil level or pressure, an issue with the ignition system, or an exhaust leak.