Sourdough starters/cultures will have their own unique scent that’s either sweet and yeasty, or sour depending on the type of feeding schedule and ratio, etc.
When a starter/culture smells like a healthy, active yeast culture, the baker knows he or she is on the right track. If the sourdough starter/culture smells like ammonia, rotten eggs, acetone, or otherwise very pungent and unpleasant, then the culture may not quite be ready yet. This is likely an indication that the bacterial culture has come to be host to some unwanted bacteria, active as they may be, and/or has produced lots of acetic acid.
This has happened to me most recently. My culture is made from whole wheat flour, which is much preferred by bacteria over AP flour. It may not be a great cause for concern, though. I think that bacteria are doing their job as they go about their business by creating largely good acids. I have baked good-tasting bread with a rather smelly starter/culture. All bacteria will eventually die in the oven while the loaf bakes for 45 minutes to 210° F at oven temps between 500° F to 475° F.
But no, I do not wish to keep smelling stinky culture, and neither does my family. I recently modified the maintenance of my culture and substituted 25% rye and 75% AP flour for all the whole wheat flour.
Searching for answers, I found two extensive posts that helped me understand better.
I now keep feeding the culture in question more frequently and hope that the smell settles down in a few weeks as the good bacteria take over to start producing the more typical fruity smells. Once the culture is better established, I count on it to release delightful aromas.
However, giving acetic acid-producing organisms optimal conditions to thrive and multiply will produce a more tangy bread.