A3 2003 Knocking/thumping

Hello,

I have an Audi A3 2003 model it’s my first car and I’m having some issues with. 

The first few months were fine and now the car seems to have a knocking/thump noise whenever I brake as approaching a junction etc so when I slow down to about 20-15mph. At first we thought it might be brakes as they were worn. I got 4 new pads and discs however it continued, all the wheels have been checked and nothing seems to be loose or anything. 

Also to add the knocking is very inconsistent sometimes it’s there and other times it’s not. Also don’t seem to be affected by hot or cold engine it literally has no pattern

any help and any approximate cost would be appreciated

thanks

Alex