The noise the debit/credit card machines in stores make when your transaction is completed. It makes an irritating sound, it reminds me of the wrong answer buzzer they use d on game shows.
I think it is intended to remind you to remove your card. But, I wish they had a better sound or voice saying thank you.
Ha... I never noticed that... probably something that I won't be able to stop hearing now!
The winners of the SBI giveaway and the next question (Butter goes on which side?) can be found here:
https://steemit.com/contest/@bengy/steem-basic-income-giveaway-butter-goes-on-which-side