tpcollins,
If they are loud or heavy is all perspective. If you ask a fire arms hunter, they will not tell you they are loud od heavy. If you ask a bow guy, he will tell you they are loud and heavy.
Fact of the matter is, they are louder and heavier than a compound bow, but quieter and about the same weight as a firearm. You hunting distance is about the same as a compound bow. The deer can hear a compound bow as well as a crossbow. String jump is a possibility on both, especially on an alert deer.
Crossbows, like anything else, have models that are louder and quieter. There are quiet crossbows if you are comparing them to other crossbows, just like there are loud crossbows.