It's the pads moving. If you put the springs in the wrong way you would get this every time you braked going forward. The springs retainers hold the pads to the front of cailiper so when you brake you don't get a clunk. But in reverse you are pushing them back and they knock on the calliper. This only happens on the steel set up as it did this on my R8. Once I had fitted ceramics I never had the issue.

Hope this helps.