I did an observation mediation focusing on sounds. It's late, and pretty quiet, so the only sounds were soft. I noted that the difference between real sounds and imagined sounds was small. For instance, there's a road nearby, and I know that cars drive on it periodically, and I would listen for them, and sometimes I'd think I heard one, but then I'd hear a real one, and it would be obvious that I was just imagining something before.. I feel like part of recognizing very faint sounds involves imaging them.