What you are hearing is intermittent slipping and gripping that is occurring in a range audible to humans. The goal is to tune this to an inaudible frequency. As others have stated, toeing the pads and cleaning the interface can help, but often, there's so much slop in cheap brakes mounted to cheap frames that there are certain conditions, i.e. high humidity, where they are going to make noise no matter what.

Good luck.