Samsung Galaxy S5 noise problem when alarm goes off?

[little-wren]

My Samsung Galaxy S5 is 6 months old and I've been really pleased with it, but over the past couple of days I've noticed that when my alarm goes off, it makes a vibrating kind of noise (not the same vibrating as when you have 'vibrate' switched on though). Needless to say, the vibrate option is turned off and up until then has always been ok. Other sounds are fine - videos, ringtone, texts etc. it's just my alarm.

Can anyone shed any light on what the problem might be please? I'm up to date with all updates.

Many thanks.