The reverse limb crossbows do tend to be quieter. One of the hunting mags awhile back did a comparison of several x-bows and the quietest by far I believe was the reverse-limb Horton Havoc. Horton also has another reverse limb called the Fury which is quite a bit louder - maybe due to design, or because of speed - it shoots at 360 fps where the Havoc shoots at 318 fps. Personally I would take 318 fps and be quieter....

All x-bows are going to be louder than a vertical bow, though. The quietest x-bow I've ever shot was the Barnett RC-150 - that one is pretty friggin quiet. But still a little louder than my PSE vertical compound.

If you are on a budget, I think Horton makes good bows. I have used a few different models over the years and they have worked well. Quality/durability is much better than Barnett, but probably a step below Excal or Tenpoint. I picked up a new Horton Brotherhood last year for $300 with a 4 power scope, quiver and arrows included. That is a pretty good deal. It worked very well for me last season. It is quite heavy and loud, but shoots fast and accurate.