Ticking is driving me mental

[Figuarus]

Guys I've got a little bit of a problem. When I drive my truck on the freeway and get it good and hot, it starts to tick once you get it back down to idle.

It comes in waves. Growing from barely noticeable to "yeah it's ticking" and back to gone over the course of about 7-10 seconds. It cycles through and it's starting to make me nervous. I'm thinking it's oil pressure but don't know how to properly diagnose.

I know that if I bring the engine to idle once it's been run hot the oil pressure reads below 15 psi. When I lean on the gas just a little with my foot the pressure Rises slightly and the ticking noise starts to go away. I haven't actually found out where the noise is coming from but I'm thinking it may be in the valve train. Is there a good way for me to know what is causing this problem?

My brother and I rebuilt this engine about 15 years ago but the engine doesn't have more than 10000 miles since it was done. I'm wondering what should my oil PSI be at normal operating temperature?