Hi,

After we flush the toilet, as the bowl is close to filling up, there is a repeated tapping/knocking sound in the wall that continues for 5-6 seconds until it tapers off -- it also becomes more rapid and softer as it goes away.

Any idea what it is?

Thanks!
Claire

Bad fill valve washer. You could try to repair it but for $14 you can simply install a top of the line Fluidmaster 400 A Fluidmaster 15543143 N/A - Build.com