whistling noise from float?

My toilet makes a whistling noise some time after flushing and especially at night when the water pressure in our area is high. I've tried to adjust the float using the screw and this works for a short time -then the noise starts again. The water level also increases and goes into the internal overflow even although I've adjusted the level to about an inch below the overflow pipe. It's a dualflush button push system. Help!