Two "pops" when I try to start my Tacoma

All,

I apologize if this is the wrong forum to post my question.

Here's the deal:

I've had trouble with my positive battery cable coming loose due to vibration while driving. (This is due to buying a really cheap battery with small posts) Anyway, every once in awhile my pickup won't start and I'll have to tighten the cable to the battery.

Anyway, last night I tried to start my car and heard "pop, pop" sound. That's two pops in rapid succession...the first was slightly louder and higher pitched than the second. (This is not the rapid clicking indicating a bad battery--completely different sound.) So, I went inside and had my wife come out and try to start the pickup while I watched. Low and behold, there were a few sparks from the positive terminal when she tried to start it. I checked the terminal and it was clean but a little loose, so I tightened it and tried again. Once more I got the duel pops--but no sparks. I tried to locate where the noise was coming from, and it seemed to be located near the alternator. I also noticed that the belt looked pretty worn.

Any ideas what might be going on?