What's that sound?

I was filling in on register while a cashier took a break. The customer was wacky. I heard a strange noise in the background and thought maybe it was on the radio or something. After a minute or so the customer suddenly asked what the sound was as if she were surprised or scared or something.

I replied that I didn't know. Then the sound stopped. The customer continued to wonder, aloud, what that sound could have been. I just wanted her to leave so I could ring out the people behind her. As she put her wallet back into her purse suddenly the strange sound started again, louder this time......her cell phone, with some kind of wacky ring that sounded like bubbles rising out of water, bloop, bloop, bloop.