Bubbles in toilet ???

I'm guessing you have a "pile" right at the base of the pipe where it drains into the tank. Being as they are only 5-8" deep it doesn't take much to partly obstruct whats coming down the pipe. Perhaps fill the tank with fresh water and let it sit a few days, then drain/fill again. Can you shine a flashlight down the toilet to see if you see the tank? When our tank "burps" its either near full or cant get air behind water.

Might be a blocked vent. Could be critters have blocked it by nesting or dying in it. Worst case would be if the vent pipe itself has dropped into the tank.

Lyle