knocking sound when braking

hi. I had a problem develop today. When slowing down gently to stop I get a single knocking sound or thud from front of car.  This can be felt through the foot pedal. It happens every time and not when first pressing the break, but when car is almost at a standstill. 

breaking performance doesn't seem to be effected. But any idea what this might be please and do I need to get it looked at asap?

car is a Mercedes e class estate  2007. E280 cdi sport. 100,000m