Bubbling in coolant reservoir tank

Usually sign of blown head gasket

unless you have worked on coolant system and have air trapped inside.
If not probably head gasket.

 

Pull the overflow line off, secure a rubber glove over the cap as well as nipple that goes to overflow line. If you can’t get glove over both plug overflow line for test. Start it and walk away. If it fills glove it’s gonna hurt. If glove stays limp your ok.

let it run 45 minutes or so

 

Combustion gasses leaking into the cooling system create pressure and bubbles, which can be seen in the overflow tank.

It’s not always but cummins run a lot of pressure, mine does..
How To Diagnose and Repair a Cummins Head Gasket - Diesel Power Products Blog
mine would bubble cold, then as it got to operating temp it’d suck the coolant back out if reservoir tank at idle then show low coolant, drive it and it’d go back into the tank.