Squeky sound from belts?

every month i clean my motor for any dust particles and oil. after all those washes i started hearing a squeaking sound coming form the belt and over the time it started to be louder until it started snowing in my city the sound became loud that pedestrians start noticing the sound i use some belt dressing and it work for several moments but when i go faster the sound comes back is a 2006 mustang everything looks in a good shape belt, tensioner pulleys