Is it normal after a breast reduction to have a "slosh" sound, like when you are water logged, when you move your arm?

It sounds like you probably have some body fluid collecting in the surgical space (seroma), although it is hard to know without seeing you.  There is a chance it will simply resolve on its own although it may need to be aspirated at the office.  Your best option is to call your plastic surgeon and he or she can tell you whether the fluid needs to be addressed / worked up further, or if it can just be watched to see if it resolves naturally.

This is a normal part of post operative recovery especially if you did not have drains after surgery.  My patients often hear this.  IT ALWAYS GOES AWAY!  The reason you hear this is that some body fluid (blood, or serous drainage) is trapped in the breast.  Occasionally you might even hear bubbles - due to trapped air in the breast.  The air and the fluid will typically go away in time. Rest and minimizing activities will help with your healing. If one breast is significantly larger than the other side you should contact your surgeon right away.