Sometimes when I'm coming up on a stop light and I hit the breaks a little harder then normal especially the rear break I hear what sounds like the bike shifting. Clunking sound from lower in the bike. I doesn't shift but it sounds like it. Anyone now what this is? The bike doesn't shift unless I shift it but does sound a lot like shifting.

 

Check your motor mounts. Have seen a few broken ones before. Also check your headset for any play, you usually feel the stem loose when braking hard or quickly, but feel it in the bars most.