Heater not working well

Hey Folks,
Hoping for a little guidance with this one.
2012 DCLB 4.0
Normally the heater in my truck would come up to temps after about 5-8 minutes of running, no matter the outdoor temp.
This year, it doesn’t seem to start to blow hot air until I rev it a bit, or start driving.
I do park on a 10ish degree driveway with the rear end on the high side. But that has always been that way.
I do hear a little gurgling behind the dash after revving it, do you think it’s as simple a burping the system with the front end in the air? The overflow reservoir is at the correct level, I haven’t really dig into it, I bought some coolant the other day expecting to find the reservoir low, but it wasn’t.

 

What does the coolant in the radiator look like.

 

Check the coolant level in the radiator not just the reservoir. If it's low you have a leak somewhere.

The heater core is a high point in the cooling system so if the coolant level is low it gets starved of hot coolant and heater performance drops.

It's common for customers to come in complaining of no heat and we end up finding a coolant leak somewhere