From D&D Wiki
If you make any noise when sneaking around in the presence of a dragon (where otherwise the space is silent), there is a 100% chance the dragon will hear it and open their eyes and see if there's anything obvious. There is a 50/50 chance the dragon will move their head around in a roughly 360 degree arc to look for any threats. It will go back to a sleep/resting state if there is nothing seen or heard further.
If another sound is made...