I just finished installing a new bathroom in my house. From left to right I have the tub, the toilet and then the sink. All three of these fixtures are connected to a new 4" PVC drain with vents for all three fixtures. The original toilet is also installed on this line directely above where the PVC goes into the floor into the cast. My problem is that whenever I flush either toilet I hear a loud gurgelling noise coming from the tub drain. I know PVC pipes are loud but that should just be in the basement where the pipes are right? Any ideas to help ease my frustration?

Got to love plastic, did you notice the plastic department is right near the ear muffs and plungers as your in for a long time of both noise and stoppages

Plastic DWV does cause gurgelling.

If it is properly installed you might hear some noise in the walls from the water splashing down the pipes, but not gurgelling.

Something is not installed correctly. A negative pitch in the drain pipes, the vents not connected correctly, maybe a test cover left on the vent pipes.