Mac Mail Sounds

Hi All,
On the whole I love the Mail app on Mac, but something has been bugging me. When I have the app open, it is constantly making the ding sound, which is normally to alert you of new mails arriving, but when I check it, there is no new mail there.
Does anyone know why it is making this sound when there is not a new email, and how I can stop it, and only hear the sound for actual new mails arriving?
Many thanks,
Dave