Would like to have your thoughts on this noise & oil pressure fluctuation

When driving my Road Toad V8 conversion with a built 360 on the highway yesterday when it was hot (98°), the temperature gauge went to an almost-hot reading (still just within the normal range), which is not surprising because my cooling system consists of a bunch of mix-n-match parts and a ho-made shroud. Since it didn't get up to "hot" on the gauge, I kept driving.

I noticed that my oil pressure (mechanical oil pressure gauge) was fluctuating more than usual. Usually it stays right around 50 psi, but yesterday when the engine was hot it would go down to about 30 and then back up to 40 AND I think I started to hear a little noise from the hydraulic lifters. Made it home OK (it ran cooler at lower speeds); the temp was fine, no noises, no oil leaks.

Would anybody like to take a stab at what was happening? I wonder if I was really hearing lifter noise, or whether I was imagining it. It wasn't that loud.