re: Any mechanics on here?

Yes.

Best bet. Drain system. Flush with hose. Refill with distilled water. Run. Drain. Flush with distilled. Fill with coolant...Run truck, burp system.

Then run truck with the radiator cap off and look for any bubbles in the coolant. If there isn't, but you're noticing some contamination from somewhere... If a beater you don't wanna spend the money to fix, buy a bottle of blueseal poor and go. Follow instructions carefully, say a prayer, may fix it.

Honestly, could just be a dirty as hell coolant system. Algae can grow in them after all.