I ran the Debian net install via floppy disks, and during process, my wireless modem light came on and the speakers emitted odd noises. I only have a cable internet connection, not wireless (the modem is of type not used anymore), so I don't want it transmitting random stuff around while installing, but I don't see anywhere during install process where I choose which type of internet connection to use. Is the odd noises a sign that this wireless modem is transmitting? Is there some way to disable this without physically gutting my laptop?

It wouldn't be transmitting anything during install. I don't know what the noises are (possibly a buggy sound driver), but I don't think it is caused by your WiFi hardware. That would have to be an exceptionally poorly engineered laptop.

As for disabling the WiFi, obviously that is not something that would be a standard option in the installer. If you really feel the need to turn it off, you could just shut it down with a command like:

Or if that is not enough, remove the modules that your WiFi hardware is using.