Musky smell

I have a 2005 GMC Envoy and i have a musky smell. The thing is i only get the smell when i am drivin with the sunroof open. I checked the 2 front drains of the sunroof they r draining fine. Is there rear drains for these also? Or any ideas where it is coming from or what could be causing the smell?

You might want to have the cabin filter checked. My mom had the same problem and after having the cabin filter checked, it fixed the problem.