Bobcat, 61inch, 24hp Koehler engine makes bubbling noise in carb when adding new engine oil

[Mrsfixit]

Friend of mine called and asked me if I ever heard of this situation and if I could explain it. He has a Bobcat zero turn with a 24 hp Koehler engine. He drained the oil and when he started to add new oil, he could hear bubbling noises coming from his carb. The noise sounded like someone was blowing through a straw. He even stated that he could see evidence of a small amount of gas somehow bubbling into the carb. Is it possible that the pvc tube could be pressurizing the carb from the crankcase somehow? Has anyone ever heard this or have an explanation that I could share with him.
thank you

 

[ILENGINE]

I suspect that when adding oil it displaces the air in the crankcase. that air escapes from the breather which it should. The breather is connect to the air filter housing which could be were the sound is coming from

 

[Mrsfixit]

I suspect that when adding oil it displaces the air in the crankcase. that air escapes from the breather which it should. The breather is connect to the air filter housing which could be were the sound is coming from

Thank you