2012 Volvo with ISX

[JMBT]

Hello, can anyone out there with a Volvo/Cummins setup tell me when you start your truck how much flow of coolant/air do you have coming from the vent hose on top of the thermostat housing into the surge tank on top of the radiator?
I just replaced my original surge tank, drain off 5 gal of coolant, swapped tanks and refilled. Tank had two new caps already installed. Then while perched on the tire looking down at the tank I noticed a lot of flow coming from the thermostat vent hose back into the tank. I have never noticed before so I don't know if it is normal or not.
Flow is while truck is hot or cold, is not affected by disconnecting the air compressor, but when I start it without the fan belt hooked up it stops completely. No mixed fluids, truck runs fine, and no lost coolant from vent side of tank. Also not overheating.
Volvo service writer says ok as long as not puking any out of the tank onto ground. Just curious. Thanks in advance.