knocking brakes A6 Avant

[Blueone]

I recently changed the front pads on my 1999 A6 Avant.
The clip gave me some headaches, but apart from that all went well.
600 miles in and at slow speeds (because less road noise I suspect) I get a knocking when applying the brakes. Sounds like a heavyish tap from both sides.
Any ideas....??

 

[enda1]

This may sound stupid but have you got them in the right way round. There is an arrow on the pads to show that they must point in the direction of travel of the wheels