This is not a useful parameter (2013) as the Pie Town link is not in operation. Someday it may be reactivated, although with different hardware.
NOISEFRQ is used to tell the system to use either the VLBA or VLA noise tube switching frequency. This is for VLBA antennas only and actually only applies to Pie Town for when the Pie Town-VLA link is working. Note that the VLA switching frequency is 9.6 Hz and the VLBA frequency is 80 Hz. This means, for example, that there will be 80 on and 80 off states per second on the VLBA.
This has no effect on the VEX file.