I have a brand new subwoofer for my car. The box is not new. The sub itself vibrates and rattles when music is on any volume. Even low. Some say this is due to an airleak. Can anyone help me with this?

Three things come to mind:
1. The surround is broken
2. There is DC to the speaker.
3. Scratching comes from an overheated voice coil. Push on the speaker and try to feel friction. There should not be any.

OK, and what do you mean by the surround is broken?
And Dc to the speaker?

Friction? You mean push on it and see if its rubbing against something else

The surround is the small amount of material that connects to the cone of the speaker to the outer edge. Basically the speaker no longer moves linearly back and forth.

The speaker consists of a coil and magnet with a small gap. If the coil gets overheated it scrapes against the gap. Push n the cone and listen/feel as u=you push for resistance or scraping.

DC is direct current. That happens when the amplifier clips. That causes DC to appear briefly at the speaker. It may overheat the speaker.