First, make sure this isn't an artifact caused by the computer. If the volume of the strange noise changes as you increase power to the speaker, and then lowers when power is reduced, there's a good chance it's a sound card, software or computer system problem. So you might check another speaker and see if the noise is gone. If the noise is still present with another speaker connected, it's a pretty sure bet this is a computer system thing.

If you're sure it's caused by the speaker then you have a defect. A rubbing voice coil will make a strange sound. It often sounds like the voice coil is "bottoming out" even when power levels aren't high. If it's the speaker, send it back where you got it for warranty replacement.