That's a pretty big project. Tell me why, pls be specific. thx.

 

Just everything mentioned so far points to a leak somewhere into the block, your truck is consuming coolant and unless you've noticed the cooling system being pressurized in a way that prevents you from pulling the radiator cap off without it spilling coolant during engine operation im willing to bet its the LIM gasket. there are many things that can cause sludge on the cap but usually that would indicate a leak in the oil cooler pissing oil into the coolant or a blown head gasket.. and those are mercifully rare compared to LIM gasket failure.

 

There are a half dozen ways to lose a quart of coolant in six months. I'm hoping I got one of the good ways because I emptied the old oil and put in the synthetic.
Only thing I've noticed so far is the oil pressure runs a little higher on the gauge.

 

Do what I did.. Ditch that banjo clamp for an ACTUAL clamp. Then the loooong 18" screwdrivers from Autozone, you can reach it from the top of the engine.

I also got it from the bottom when there was no shielding in place by using a stubby.

 

There are two.. External leak or internal leak, sir. (People say a radiator cap no longer holding pressure can do it too and it is cheap insurance to change that cap at some point, odd 15lbs not 16lbs units. But I'm not sure I have ever seen this. Is possible, though. That could be considered either one, so, two and a half.)

I am not trying to be mean. But. In 2014/2015 or so, I bought a Blazer with a head gasket leak (absolutely trashed mudding) and I became very, very familiar with things. Long story short: With everything I tried, that engine never did run correctly and it needed another engine. But I learned all about ways coolant can be lost, condensation vs normal on cold mornings... adding in to the radiator vs the other tank, usually the radiator way is better, the overflow tank is less crucial, .. a bunch of things. That engine also sounded like a conga drum but hey, if your system is not being overpressurized from exhaust gases and blowing hoses one by one..

Sounds like a slow, internal leak. Makes coolant a funky brown color... could make it other colors too but. I just switch to green coolant..

 

Epilogue: after a year of adding water periodically, the mystery has been solved. Turns out it was a slow leak in the radiator that did not reveal itself until yesterday when I walked out of the car and heard a hissing sound. Steam coming out. Was not a slow leak anymore!
The leak was on the top of the radiator near the upper hose so I suppose that explains why there was never a puddle underneath.