What could be the reason for this sizzle when braking?

I recently bought a set of used FFWD Tyro carbon rim brake wheels used, but in very good condition. They came with the stock/recommended SwissStop Black Prince brake pads.

When installing the pads, I tried to make sure they were right in the center of the brake track and at a slight angle (using a rubberband & squeezing the brake). However, when I put pressure on the brakes, it makes a sort of sizzly, squeaky sound.

Is there anything I could have missed / glossed over in terms of inspection or installation?

Or has anyone got the same set of wheels?

Thanks a lot for your responses!