This truck is loosing coolant but I have no wet spot anywhere inside, outside, or engine bay that I can find.
From time to time I can hear the heater core “Gurgle”.
I took the cap of with the engine cold and let it warm up some and the coolant seemed to go from a foam to small bubble’s to nearly nothing at all.
I looked at the oil and no coolant in the oil I can see.
No smell from the heater core.
Transmission is a standard.
I will assume you crawled under the truck and did not see any external leaks. I’ll also assume you somehow made sure your heater core is not leaking.
How’s the engine oil level?
Way too high?
These engines are quite well known for having the lower intake gaskets deteriorate . . . to the point that you lose coolant either into the crankcase, externally . . . or both
Here’s my recommendation . . . and you’ll make a mess
Drive the truck for several miles to get the fluids nice and hot
Place a deep pan under the engine oil pan
Drain the oil into that pan
Is it all one color?
Or does it look like starbucks moccha frappucino?
If it’s the latter, it’s almost guaranteed your lower intake gaskets have crumbled into dust. Stop leak, blue devil, bar’s, etc. won’t fix that, by the way.
If truck has an automatic transmission, pull the transmission dipstick and check the condition of the fluid.
Tester
Engine oil is dark but it might have been a little high not much.
Its a standard
Could be a head gasket - especially with the gurgle of air in the heater. Compression gasses passing into the coolant and coolant being sucked out the exhaust rather than ending up in the oil. Put a white paper towel over the exhaust when the truck is heated up. Does it smell sweet? That would be coolant and likely mean a head gasket (or 2) is in order
I like your thinking . . .
but it has been my experience that it’s usually the lower intake gaskets . . . on this particular engine
In any case, you’ll have your answer when you pull the manifold
The manifold’s gotta come off either way. Hopefully the OP will find it’s the manifold gasket.
Pulling heads on a truck usually causes me to break out the milk crates and boards to stand on as I’m not quite tall enough (or arms long enough!) to reach all the fasteners.
I’ve known a few guys that seriously messed up their lower back when pulling heads
Lifting was okay, but when they turned 
One guy tore his lower back muscles clean off . . . he was never the same again, even after he was declared “recovered”
I’m not very tall, either. And I regularly use a small step at work, when doing some engine work. I also use hearing protection, unlike some of the “manly” guys . . . I’d rather keep what I’ve got, so to speak. I’ve already got tinnitus, but my hearing itself still seems to be okay. I’d like to keep it that way.
That simple move brought me to my knees in pain - and I was only moving a coffee maker!
Your comment about torn back muscles brought tears to my eyes just reading it! Wow!
How much oil goes in this engine? I seen 6 quarts then someone said 5 but its showing over fill with 4.5 quarts?
I also noticed the radiator, heater core, thermostat, and half the tubes are pretty new. I only had the truck about a month.
The poor guy was literally bed-ridden for months
He could only get up, if he pulled himself up with assistance.
Then again, he was one of those guys that was too fast for his own good. He always did everything really fast, and wasn’t being careful. That story with his back, was only one of the the times he hurt himself due to carelessness.
Years ago, I worked for UPS . . . only for a few months. But the one thing I took away from that job was proper lifting techniques.
Plenty of times on this forum, I’ve said my back was messed up, deformed, actually. But not because of work.
Born that way. And I HAVE hurt my back at work, but it’s never been due to improper lifting techniques.
All I can small is a damp paper towl. nothing sweet but pretty wet.
Also its leaking when the engine is off as well. I can fill it up and walk away a few hours and it like 3 inches gone from the radiator fill.
If you are absolutely certain it’s not dripping externally or being burnt. . . water pump, heater core, huge plumes of blue smoke, for example . . . then I’d say the coolant is definitely going into the crankcase
Think I found it it was hidden behind the AC bracket. Rather a leaking freeze plug or the left side of the water pump is leaking into the plug making the plug look like its leaking.
I have an issue with my 4.3 as well, 178k miles, oil looks good. engine is not using coolant or overheating. Although in the past 2 weeks ive had lots of white smoke come out the exhaust. once while driving for about 5 minutes, Then once when i shut the vehicle off, that time was very bad. The only code it is reading is PO174(fuel trim system lean bank 2)
any idea what to check.? I dont think its leaking head gasket or 02 sensors. i know for sure its runnnig lean. Maybe maf sensor ?
Use an inspection mirror
and look behind the water pump pulley from the under side. A slow seep will leave a yellowish scale at the weep hole where the coolant escapes when the pump seal is beginning to fail.
Your lower intake gaskets are probably shot, allowing coolant into the crankcase
That would explain lean fault codes AND the white smoke