The yellow highlighted box is indeed a muffler as 600 has correctly pointed out. It is used to reduce engine intake noise and it also acts as a direct replacement for the standard intake grid by protecting the engine from ingestion of foreign material. The muffler is becoming more common on civi gazelle's these days. There's only a small power limitation when using the muffler that would only take affect when operating hot and high. I am uncertain as to the possibility of the muffler being used as an anti icing shield but it is recommended when flying in falling snow to use the regular grid, otherwise you are limited to 1hr intervals of flying time.