What am I hearing?
It is likely that the noise/static/interference you’re experiencing is being caused by a background RF signal from an external source (ie: broadcast TV or radio stations, other wireless microphones, etc) on the same frequency as your wireless system.
This can be confirmed by turning off the transmitter and checking the receiver’s display. If there is an RF signal showing on the receiver's display when the transmitter is off that indicates that the receiver is picking up an RF signal from an external source and the receiver is trying to tune this external RF signal which will sound like a broad white noise.
One of the most important steps for proper setup is turning on the receiver first and checking what it sees in the absence of your microphone signal. If the receiver sees an RF signal being generated by an outside source (ie: noise) then you need to retune the system. Depending on which system is being used the exact steps on how to change frequencies will vary. NOTE: When using multiple wireless systems, the frequencies need to be coordinated together to ensure that they do not interfere with each other.
Once a new frequency is selected get back to the receiver's main display. If there is no RF registering, then this new frequency is a "clean" frequency on which to operate.
Keep in mind the RF environment changes from location to location, so it is important to always check the receiver for RF levels before turning on the transmitter. It is best practice to always do a frequency scan when using equipment in a new location.
The RF environment is often constantly changing so a frequency scan is really only completely accurate for the exact time that the frequency scan is done (ie: a frequency scan done on Wednesday night could yield different results to a frequency scan done on Sunday morning). Ultimately to ensure a frequency scan is still valid you want to turn on the receivers and leave the microphones off. In this state the receivers should be showing no RF signal to indicate that they are on a clean usable frequency. If you turn on the receivers (with the microphones off) and you are seeing a lot of RF activity that indicates that something in the RF environment has changed and now the frequency which was previously "clean" (and hence usable) is now "dirty" (ie there is an external RF signal on it) and you will need to set the receiver to use a different frequency.