Macbook Croaking/ Clicking sound

I've noticed that every couple of seconds my Macbook makes a croaking sound. It seems to be coming from behind the number keys at the top of the keyboard. The sound is only noticable when in a quiet room and either when opening the Macbook from sleep or closing the lid and putting the Macbook to sleep or while doing a light task such as word processing. I wanted to know if this sound is normal and if not where it could possibly be coming from.