Locate your rear burp line to the exhaust side (rear side that goes towards the ignition coils and where the transmission and engine meet - that gap you'll find a hose from your coolant tank - that goes to the rear exhaust cylinder head)
The hose needs to be in the upper port of your coolant tank - and that line is supposed to slope upwards - and lets the hot air and steam trapped in the fluid back there - to vent back towards the coolant tank.
If that doesn't happen the trapped steam builds up and displaces fluid that can cool that part of the engine and will eventually force the head to warp and even blow a gasket. The bubble you're seeing may be due to the fluid displaced is now showing that air is trapped in the block at the thermostat so every time it opens - it burps out this trapped air.
If you replaced the thermostat - that bubble valve or hole on the flange of the thermostat that seats into the block - that hole or valve needs to be oriented to the top of the thermostat when it's inserted in. The trapped air can then leave without having to wait for the valve to open when the engine gets hot.
The bubbles you see may be an indication that either the head is ready to fail, or it's close to it and that return line may be plugged or the bubble valve in that rear port is damaged and will need to be fixed or cleared out to free it up and work again.
