Are old muscle cars supposed to sound like that?

Every time I see an old Charger or Challenger or Mustang, etc, the (presumably souped-up) engines always sound like sputtering motorboats. They seem barely able to idle without stalling and if I happen upon a guy just starting it up he immediately starts revving it to keep it going. Now mind, this is the result of MANY years of observation, not just some guy I saw once. I always umed these cars have been hot-rodded and modified and that's the cause but why do they so often sound like out-of-tune motorcycles afterward?