Stomach gurgling?

[Serpentine]

I've been treating my snakes for mites and today when I gave them a rub down with mineral oil, one of my snakes seemed to have a bubble around it's stomach (inside), when I would rub over it it would make popping/gurgling sounds... is this normal? Or am I soaking them too often in water (daily about 30 mins)... I dunno, just guessing really, new to all this,

Thanks