Since I ride mostly in the desert, I have a aluminum skid plate on my bike. And while it does a great job of protecting the cases from rock damage it reflects a great deal of engine noise.
I read somewhere that placing a piece of foam between the skid plate and the engine would cut down this noise. But at the same time it seems that it could potentially block airflow thereby increasing heat which is not a good thing.
Since I had an old wetsuit that I don't use any more, I cut a piece of rubber from it to the shape of the inside of the skid plate and glued it in place with liquid nails. Anyhow, after the glue dried I bolted it back onto the bike. Since the rubber was only 3mm thick, it didn't cause any fit problems.
When I started my bike, I was blown away by how much less mechanical noise there was. The difference is amazing. I think all the skid plate manufacturers ought to do this. Similar results can probably be realized with other types of rubber as long as its not too thick to pose potential fitment problems. If you try, this I'm sure you'll agree the results are worth the effort.