The phrase "tapping sound" refers to a repeated and light noise that is made when something hits or touches a surface gently, creating a rhythmic sound. Full definition
Mine looked great before it went in the oven, indeed when I took it out it looked good but the little tapping sound of the seeds hitting the floor as I carried the loaf from the oven to the counter was the first indication that the seeds had not stuck on. (madebyjayne.com)