Try switching to a channel that doesn't have the noise - the problem with radio mics is that they cannot always squelch out harmonic frequencies for high power transmitters, such as taxis and so on.

What you are hearing is interference from outside sources... even though the transmitter isn't on the same frequency as your receiver, which is why you don't hear the actual transmission, there are other frequencies carried which can open the squelch and/or make horrible noises.

When adjusting your squelch please bear in mind that you could end up reducing the range of your system to a point where it becomes unusable!