The Gemtech M4-96D is an awesome suppressor. It is great because of the QD mount and its durability for full or burst fire. The Gemtech M4-02 is great as well and lighter weight, but not rated for full auto and very limited burst fire. The loudest sound IMHO is the bullet flight noise as it breaks the sound barrier. About like an unsuppressed .22lr or maybe a little quieter. I have heard that the AAC Ranger is pretty good too, about like the M4-02, but I have no experience with that particular piece. Check out this link for anything you want to know about suppressors
http://www.silencertests.com