My 1998 I30 with 241k miles has some excessive road noise from the cabin especially at higher speeds...if on the highway and it's raining outside you can almost hear water whipping up around the back wheels clear. Now my underside/pinch welds does have the typical expected rust these cars have, but there's no noticeable rust or holes at the floorboards. Not sure how common this is...it's my daily driver to work (I'm in it alone) and just care about it more from a safety perspective...don't want to fall through the bottom of the car while I'm driving..yikes. But yeah I only hear it through the back of the car; not the front. My guess is a portion of the flooring under/around the back seat that is not externally visible has rotted away.

 

Not quite sure why you seem to be associating excessive road noise with the possibility of disintegrating structural components as opposed to the likelihood of disintegrating tires or suspension components.

 

That was my biggest complaint with the car, especially on wet roads. Try a few coats of rubberized under coating in the wheel wells and on the body. Get it up on a lift will help you get everything coated nicely.