I recently added a new bathroom and in the sink and shower we hear air popping
Noises through the drain traps after running the water. This lasts several minutes and
Make single popping noises every few seconds. How can I fix this?

Did you vent the new plumbing? Sounds like an inadequate vent to me.

I hired a plumber to install the new bathroom. I think he just attached the plumbing
To the existing lines. So do I need another vent?

Stew's on to something here. If the sound you hear is gurgle sound then your new fixtures are not vented. Call the plumber back. Good luck, Tom