I'm sorry, perhaps it's me but I couldn't hear it even with the volume right up.
Was it a rumble sound?
Sometimes it's better to go on body language as rumbles can mean different things and the difference between the types of rumble can be subtle.
Does your guinea pig appear to be tense at all, or look relieved to be back in the cage when you take her back? Or does your guinea pig stretch out/lie a little to one side? Is your pig fidgety, does he nudge you or nibble at your clothes?