During a flight, you often hear the "pinging" sound that accompanies the fasten seat belt signs coming on, occuring on its own, sometimes in pairs, for no specific reason. I presume it is some sort of signal to the cabin crew from the fight deck. What is being signalled?
The 'ping' sound comes when someone presses the 'flight atteendent' button above their seat - it's to alert the cabin crew to see which seat's light has illuminated. Once a crew member sees it, they press a 'cancel' button on the nearest seat panel to them, and the ping sounds, again, alerting the remaining crew that someone is dealing with the passenger concerned.
As andy says, it may be the passenger call buttons causing the pinging, but it could also be a pre-arranged signal from the flight deck. The airline I know uses that signal shortly before landing to tell the cabin crew to sit down and strap in themselves. That would not qualify for "often" though.
If you mean the noise just before the flight when everyone has to put their seatbelts on I always just thought it was to draw your attention before the captain or attendant announces that you should do it.