the level, on 25k wideband normal max deviation is 5.0 khz. voice itself is usually about 4.5 of this and PL is .2-.5khz of deviation. if they have it set to say .8khz of PL deviation then you hear it as a hum or tone. for narrowband these levels are cut in half but the principle is the same, too much PL deviation and it no longer is a "subaudible" tone.