I added a shower where there wasn't one. I joined on to my copper with the pex pipe using the quick connectors. After I did this I noticed that when I use the bathroom fixtures there is a rattling in the pipes only for a second right after the water is turned off. What is causing this?

First, go to that bathroom and Close and Open all angle stops ( under sink, under toilet ). See if that rattling noise continues...

Back to you... Milo