Julie, What you are no doubt hearing is the pipe behind the wall. When water is called for at the lav. upstairs the pipe which is not properly secured in the wall moves and bangs against the inside of the wall.
To make it right the wall will have to be broken into from one side or the other to secure it, per code, to the studs in the wall.
You should not have to put up with this noise. The best time to deal with this sounds like now rather than later when you will be footing the bill.
The Local Plumber
Tustin, California http://www.TheLocalPlumber.com