Is the transmission switch clicking or the relay?
I don't have a '72 wiring diagram. Could the NSS be connected to part of the ignition key buzzer? or other saftey interlock circuit under the dash? You mentioned under the sound is under the dash, and the starter relay I think is in the engine compartment (and is should not click when key is not in the start position anyhow.)
If the switch in the transmission is making a noise, check that there is a gasket between the switch and transmission case. When I got my trans from Keisler the NSS was not working. Installed too deep into the trans because they forgot the washer and it smashed the end of the NSS. Had to replace the damaged switch and install with correct washer for spacing.
