Anyone that can help me I would greatly appreciate it. I live in a 2-story condo and for the past 2-3 months, every time the water in our upstairs bathroom is turned on, a noise that I can only describe as a "knocking" sound starts and is fairly loud. It can be heard in the bathroom and both bedrooms that are adjacent to the bathroom. This "knocking" continues for as long as the water in the bathroom shower or sink is running and even continues (although at a slower pace) after the water has been off for 20 min or so. This sound also occurs in our walls when our neighbors turn on their upstairs bathroom water. The problem happened about 8 months or so ago and eventually just stopped and is back again. Does ANYONE have ANY idea what this could be, its driving me crazy.

Thanks in advance.

It has something to do with the water pressure and I think air in the pipes. I think it is mainly the hot water that will do that.
I am not a plumber but here is a site that might help.

Howstuffworks "How To Fix Noisy Pipes"

This could also be caused by a facuet cartridge or washer that came loose in the faucet. It vibrates as the warter passes by.