I know this is almost 2 years old but I have the same problem with my 2018 ATS. Did you ever get it resolved?After a heavy rain or after I wash my 2016 ATS and I put drivers window down a couple of inches. I have a popping noise coming from my sunroof.
Cadillac says that it's the water in the track getting pushed to drains and the noise is normal. Anyone else having this problem?