Quite a few aircraft have a design feature that allows crew selection of 'No Smoking' and 'Seat Belt' signs to Off / Auto or On.
Off and On are obvious!
Auto can mean No Smoking signs extinguish with gear selected up and Seat Belts when the flaps are fully retracted or at a pre-programmed altitude (10,300ft, for example)
In this day and age of No Smoking, the NS signs are left in the On position. Depending on the weather on the climb out routing, the belts will either be at On or auto for take off.
As mentioned, the sound you heard could be a signal to the cabin crew that it is safe for them to move around OR the belt sign being switched off.