The Z I just bought came with a custom exhaust system. I haven't been able to get under it properly and take a look but from the back of the car I can see it has no mufflers in the back which is most likely why it is stupidly loud. I noticed something strange though. There are these 2 pipes coming off the exhaust tips that don't look to me like they have much of a purpose. Can anyone tell me what these lateral white pipes are? They have closed ends, don't touch each other and don't appear to have hangers on them...

What you have is a cheap attempt at a closed end resonator otherwise known as a Helmholtz Resonator, these types of resonators remove certain frequencies of resonance or drone. My e39 540i has one from the factory and a number of higher end exhaust systems use this design.

Depending on having the proper area they can remove objective noise or enhance it by multiplying the wave strength. Sound bounces off the dead end to come back out into the other pipe to interfere with other waves (quieter) or to line up with them timing wise (likely louder) to maybe help exhaust flow by increasing inertia extraction. The difference is in the length and area and where located in the pipe length.

Some mufflers work the same way, they split exhaust flow into two pieces then aim them at some point directly at each other to break up sonic wave noise by colliding.

Why you often see those funny little pockets of deadspace in some of the OEM fresh air intake ducts. My Versa has like 3 of them in that duct. They are low rpm drone absorbers.

^ Correct, what I should have said is they "attempt" to remove certain frequencies, when they are properly designed.