The plumber finally made it to our house at 10pm where he proceeded to snake the pipes for 2 1/2 hours.
Our dear, lazy rental agency hasn't snaked the pipes in awhile. The plumber said it was the most roots he's seen in a very...long...time. We would've fallen asleep, but the pipes were too close to us and the ringing kept us awake (see how small our place is).
All's well, that shower's well!