PM471 USB Microphone Files Download
That whistling sound is called Audio feedback and caused because of an endless loop caused when the mic picks up the sound of the loudspeakers (stage monitors or pa speakers) and sends it back to the loudspeakers to amplify and pick up again, resulting in 1 particular frequency getting boosted depending upon the resonance of the room, the speakers, etc.
Ways to correct /avoid feedback?
1. Face the speakers away from the mic or place them farther away.
2. Lower their volume or mic gain
3. Use unidirectional cardioid mics which have a rear rejection that means no pickup from back side. Then you can use stage monitors facing you in the middle of the stage. And the mic won't pick up them.
4. Ring out the speaker - deliberately create feedback by taking the mic closer to the speaker. When feedback happens, check the frequency on a real-time analyzer and cut it using channel eq or graphic eq.
5. Use a feedback reducer