I am seeing a good many young puppies, 3-6 months old, who are moving backwards away from you, snarling, growling and even biting. The last thing a puppy that young should be thinking about is snarling, growling or biting aggressively, puppies should be eager to meet everyone.
Sometimes this is a genetic issue, that means you really need to know as much as possible about the dogs your new puppy comes from. Meet the parents of the puppy on one of your visits to see the litter. If you are told that one or both aren't friendly that would be red flag #1. If you do get to meet the parents and they appear shy or stand offish, that would be red flag #2.
Sometimes young dogs have had bad experiences with people, by accident or unfortunately sometimes on purpose, and they are truly fearful of certain types of people, or all people due to these experiences. These puppies will be shy acting, trying to hind behind someone they trust or under furniture. Don't allow people to force themselves on a puppy who is acting this way. Puppies a little older will start barking or growling as they back up or try to hide. This behavior most often is fixable but requires professional help, and a good bit of time and work. If not done right, the puppy will learn to distrust all humans and the problems will get much worse. Get help as early as possible for puppies showing these behaviors...as they develop mentally the behaviors will get worse and will be much harder to fix.
Keep in mind, don't purchase or take a puppy on impulse. Make many visits watching the puppies in different environments with different distractions. If buying from a breeder research the pedigree of the litter to see what working titles the parents and grandparents have earned. If no titles, then find out what you can about what the previous generations of dogs have been used to do, herding, hunting, protection work, etc.