External Hard Drive Beeping?

I bought an external hard drive a few months ago. It's a 250gb Maxtor in a USB enclosure. It has worked fine, but now when I try and turn it on I get a weird beeping like noise and then some crackling sound like a modem speaker. I opened the enclosure and there doesn't appear to be anything unplugged or visibly damaged. The sound appears to be coming from the hard drive, as the enclosure doesn't appear to have anything that would produce sound. I ume this sound means something, but I don't know what that could be? The computer doesn't detect the hard drive anymore, and this sound keeps repeating every few seconds.