More than likely it's just a bushing. Which, in the summer would swell a bit and start rubbing the frame and squeaking, and when it rains water gets in between the bushing and the frame and acts like a lubricant.
It's not shit stuck in the wheel, it's a slight lack of lubrication. If it's a balljoint it could get serious, but if it's a bushing it's just annoying sounding mostly.