It depends on what stage it's in. Right after a feeding, the starter should smell pleasant with lots of yeast aroma, similar to fresh dough. There should be just a slight tangy scent, similar to yogurt. If the starter is overripe or hasn't been fed for awhile, it will smell sour, even acidic or vinegary, and may even smell a bit of alcohol or acetone. This is due to the build up of acetic acid and lactic acid.