Reducing Noise through Exhaust pipe
The noise you associate with exhaust is due to the rapid expansion of air during combustion and to the turbulence of its high speed exit from the engine. Smooth out these pulsations, reduce the turbulence of the exiting gases, and this will silence the exhaust noises. The usual way of reducing the noise of engine exhaust is through the installation a muffler or silencer.
A muffler works by providing a large chamber in which the sound of combustion and turbulence are reduced. The quality of the silencng, therefore, depends on the size of this chamber and on the downstream resistance the gasses hit.
You could modify the silencer by putting fibreglass packing inside the muffler, this would need replacing every once in a while though as the oil will clog the exhaust up and slow the engine down.