Constantly?
Is your device in a wallet-type case? Do you carry any cards in that case? Anything else in close proximity to your device?
I'm thinking you have NFC enabled (Settings -> Wireless & Networks -> More...) and you are hearing the notification sound which happens anytime an NFC-enabled card or fob is in near contact with the phone. European credit/ATM cards have NFC chips in them as do the access cards or fobs used by many office buildings, parking lots, and so on.
Not to mention other NFC devices. For example, if you have NFC enabled on your phone and your tablet and touch their backs together...