Abstract: An insert assembly for a muffler of a high-performance motor cycle, having an insert with a filling of compressed heat-resistant, sound-absorbing material mounted on a removable hollow tube having an inside diameter greater than the outside diameter of the perforated core of the muffler. The filling includes a bulked-up continuous filamentary material. Once the insert has been introduced into the muffler by sliding the tube along the core, the tube is then withdrawn leaving the insert behind in the muffler. Removal of the tube allows the filling to relax towards its intended density. The first subsequent exposure to hot exhaust gases causes certain portions of the insert to degrade, allowing further relaxation of the filling to assume its intended density. Further, the tube precludes potential damage to the insert during its introduction into the muffler.