Enclosures are used to attenuate noise produced by a high decibel producing device, such as a gas turbine engine or other rotating machinery. However, enclosures that achieve high Sound Transmission Class (STC) ratings are generally expensive and immobile, whereas inexpensive and mobile enclosures are generally incapable of achieving high STC ratings. Accordingly, a composite noise-attenuating panel system is disclosed that can achieve the high STC ratings associated with immobile, site-erected enclosures, using subpanels that are separated by an air gap and an internal filler (e.g., mineral wool), while maintaining the weight, form factor, and ease of use associated with lightweight, modular mobile enclosures.