Question about the volume of my box being too small.

Hey guys,

I just got a new Sundown SD-2 10" sub and it doesn't sound as good as I thought. I can hear a kind of muffled sound that sounds like it's coming from inside the box. The sub plays fine out of the box, makes no un-normal sounds. When it goes back in the box, and the gain is turned up about over 1/4 it starts to make the noise.

Now my box is .48 cu ft and .4 with the sub in. Sundown recommends a sealed box @ .5 cu ft. Could this just be the lack of size causing it to be weak and/or the noise?