My 1991 f350 7.5L V8 is making a clicking sound?

I Drive a 1991 Ford F 350 7.5 liter v8. There's a clicking sound coming from the engine block it sounds like from the right side of the block on the top from the 2nd to the front spark plug. As I speed up the sound gets faster and as I slow down the sound gets slower. I also noticed my oil pressure get higher as the clicking sound lasts longer. When I first start up the truck and head down the road the sound is not there but after about 5 to 10 minutes of driving the sound will start and slowly starts to get louder and louder. Once I shut off the truck let it sit for a little bit and then restart it the sound goes away but it always comes back after about 5 to 10 minutes of driving.

If it's full of oil, sounds like a bad valve lifter.