As has been said the hum is your threshold, the CTX is a threshold machine. You could turn it down so that it is longer audible but then you would also have no idea of the iron present.
Remember that when the machine nulls over iron it isn't actually detecting until it recovers, so the more iron contamination you have the slower you need to swing to allow the CTX to recover. If you can't hear the null then you could be happily swinging away all day on a heavily contaminated site and not actually detecting anything.
