I drive a 2004 G35 Coupe 6MT.

Last Thursday(?) it rained in San Diego and while I was on the freeway, I could hear a whole lot of road noise coming from the rear of my car. The sound was as loud as if there were no rear seats, carpeting, or anything. Just straight clear road noise almost similar to having the windows down, and sometimes when I drive through a big puddle, I could hear it all splashing on my under-body.

Is this normal, or is it a seal leak problem or is there supposed to be some padding to drown out the road noise? I'd investigate it myself but of all my friends and family, my dad being too busy, I'm the only one that can drive a MT, so I can't sit in the backseat and simulate it. Plus, I don't know if rain is a big part of it or not but it's not raining anymore either.

 

The amount of tread left on your low-profile tires may be the culprit. The less tread, the louder the road noise will be, as a general rule.

I've yet to find a bluetooth earpiece I can use with my phone in my coupe. They're just not quite loud enough. As much as it pains me to type this, I almost wouldn't mind ripping the carpet and backseat out and adding a layer of dynamat if I knew it would fit and be effective. Since I've pretty much given up on this car ever being fast (not interested in sacrificing reliability and $10k for a twin turbo system), I'd probably enjoy it a lot more if I could quiet it down some inside.

My car, at 70 m.p.h., registers 76 dB while driving on a newly-paved road. Normal conversation is about 65 dB, and loudness is generally perceived to double with each 10 dB increase. This means the interior of my car, at 70 m.p.h. on a new road, is about twice as loud as a normal conversation.

I haven't run my sound meter app on the typical interstate around here though. I suspect I'd be in the 85 dB range on a "normal" road.

 

^+1
tire wear is a huge factor. Ive had this issue untill i replaced my tires and got the shockwave exhaust.. Now all i hear is a beastly growl

 

That'd make sense, but I just got two new tires on the rear though.

My little rant on my G35's cons (lol):
I completely agree with you about the G35 being fast and it's lack of quality in some areas, such as the sound levels in the cabin. I mean it comes off as the luxury edition of the 350z but it can't compare to a other luxury cars like Lexus or BMW. The 3.5L V6 sounds bad-*** and all, but comparing it to the Genesis' 3.8L V6.. it's a joke. I'm pretty sure Corollas and Altimas have quieter cabin noise. I could blame the noise on the trunk, but the trunk is tiny as hell. That's not the only noise problem by the way, I get this windshield/dashboard rattle and the bolts holding the window hardware together is loose or something.

 

Your brand new tires could still be what's causing the unwanted noise! Have you ever had a 4 wheel alignment? Now that it has quit raining, do you still hear the same noise? Are you totally sure it's your rear tires and not the fronts? Check your front tires, are the insides excessively worn? Sorry for all the questions, noises are a tough nut to crack!
Gary