Alternator or timing belt- not sure

My car had begun making a noise lately, similar to a manual transmission when you are gearing down from one gear to another. Whenever I slowed down, it would hum as it was slowing down. The engine also made a rattling noise, like something was loose and hitting something inside the motor. The battery light came on a few days ago and stayed on while I was driving. When I checked the owners manual, it said that if that happened it was the charging system. My car died on me the other day. All of the lights on the instrument panel came on and I pulled over. I had a two truck come to tow it, and the driver jumped it and said it wasn't the alternator, it sounded like the timing belt. It can be jump started with cables, but won't stay running even with the cables still attached. Any ideas on what it is and how much it could cost to fix?