With a little luck just a loose alternator belt. With engine off see if you can turn alternator pulley fan blades. If you can turn them belt is loose and just retension alternator. Problem gone. But if you cannot turn the pulley with a little pressure borrow voltmeter if you do not own one. With engine off read voltage across battery terminals. should be 12-12 1/2 volts. Start engine and measure again voltage should be higher by a volt or so. But again I just suspect belt a little too loose by your description.