Is there a radio frequency that is sure to be quiet?
From the document referenced by mongo,


  Frequencies (MHz): 121.425 - 121.475Use: Band Protection for 121.5
  Frequencies (MHz): 121.525 - 121.575Use: Band Protection for 121.5


It seems that these frequencies exist only to be silent space around Guard/121.5.

No one should transmit on them, and you should probably hear total silence there.

I think these are an excellent place to look for quiet space.