hollow knock knock knock braking noise

I am getting ready to put the car away again for the winter this weekend. I have been daily driving my Barracuda the last couple months on nice days and a weird noise has developed when I brake. I notice it when my windows are down and there is something on the passenger side of the car like a curb to reflect the sound. It is kind of a hollow tapping or knocking noise as I brake. It almost sounds like a stone in the tread (but more hollow sounding) and has become louder the last few weeks. It seems to come from the passenger side front and slows as I slow. Wheels/hub are cool to the touch and lugs are tight. I will pull the front brakes and bearings in the spring (as I normally do) but wondered if anybody might have an idea what this might be. 1968 Formula S with original drum brakes. Stops very well especially considering I am still running drums at all 4 corners. Just looking for suggestions if it is something normal.