Remove the keyboard and other covers, remove the cpu heat sink and start checking. Anything lose? Like Trash80toHP_Mini said, a loose card can produce that. Anything more could mean a bad audio chip or speaker. Connect another speaker to the 1400 speaker and see how it sounds. A loose or frayed speaker wire can also do this, so check your cables as you take apart and reassemble the 1400.

You can get a "new" 1400 speaker cover from ebay for a couple of bucks. Its ABS Plastic, so mostly everything you put to it will discolor it. Best to use ABS Clear Piping glue and use it sparingly. Then some epoxy on the back along the crack about a 1/4 inch to each side of the crack.