only when I change pads. With brand new pads there is some vibration, but it goes away after some miles (2-3 laps). After that they are smooth as silk

 

I had this before I went to brake ducts under high speed breaking from +105 down to 45mph. I believe it is the smearing of the brake pad material due to over heating. This can also occur if the pads are not bedded correctly. But even with proper bedding I was getting the vibration. That is now gone. N tech use to sell this duct system but no longer seems to have a site open. You can also make a set. Search for picturs of the ones built by different members here.

 

so for street driving, you don't see this kind of vibration right? Only extreme braking on the track?

 

That is corect. If it is doing this on the street under light breaking, then you probably have a warped rotor.