If your brake pads are fairly new, you could be experiencing noise from metal fibers in the brake pads. Many companies manufacture brake pads with a high consistency of metal fibers, some that might end up being close to each other, which create a squealing noise. Brake pads will typically wear past this point if you ride on them long enough.
If you've been hard on your M5 brakes or have been using them for a while (25,000-60,000 miles), its possible that you were experiencing loud noises from a pad wear indicator per:
https://nubrakes.com/blog/7-signs-of-unsafe-brakes/