Is it a rattling noise or a buzzing noise? It might be a damaged speaker, something caught inside the speaker, or a loose connection. In any case, you're probably not going to be able to fix it without disassembling your netbook to get to the speaker assembly. How easy this is depends on the model, and if you're not comfortable working on computers I do not recommend it. If it's bothering you that much you might want to check and see if this is covered by your warranty, if the netbook is still covered by one. .