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?

Probably too small of an enclosure, the sub isn't able to reach it's full potential so it sounds muffled.

That's really what what I'm thinking. I've never experienced this before, but I did built the box for a shallow mount Type-R then went with the SA