Yea that sound usually comes from a bad ground but also the battery and or alternator can do it too as you have found out. If I were you I would have invested in a dry cell battery. Those things a great! Seriously though, have your alternator tested at Autozone and I'll bet more than anything that its still perfectly fine. Waste that money on a 1 farad cap or something like that depending on what type of power your system is pulling.