If its only under hard braking and all else is ok I’d check your tire pressures.
If its all the time, bleed the lines on the left side and check for smooth movement on the slide pin and pistons. Its not much to take the caliper off and see if they are retracting easily.
Edit:
2nd on the did you clean the rotors before installing them? Anyways since you (should) are taking the calipers off just hit the pads with 60-ish grit sandpaper to freshen them up.