If you have cast aluminum wheels, suspect corrosion around the beads, especially next to any clip-on wheel weights. You can verify that by spraying watered-down dish soap around the area and watching for bubbles. Usually a good scrubbing with a stuff wire brush, (after the tire is removed from the wheel), will clean them up, then the tire installer can coat both beads with bead sealer. That's a tar-like coating that fill in the voids caused by corrosion that the air sneaks out through.