Everyone is talking about dynamat, etc to line their trunks with to cut down on the resonence from their exhaust.
That is all good and well, but those products are designed mainly to stop vibration by increasing the weight on the panel they are attached to. Yes they will cut noise levels, but since the resonence is due to the trunk more than the exhaust a foam / poly asorber should be used.
You may also find a thing and dense carpet pad that can be cut to size

Commercial products such as
http://www.b-quiet.com/hliner.html

just throwing it out there.