Don't shake it! Normal and desireable, the tube over-bore fits rechargables and other cells that may be a bit oversized. If you wish shim bore with an index card or such.
Like if I don't do the rattle fixes both of you have suggested, will it be harful to leave my flashlight as is (to the light or batteries), with the rattle?
The slight rattle was something I learned to accept--it didn't occur to me to do the shimmy thing with an index card. Cut one up to fit in 2 minutes and the light is completely rock solid now. Thanks!
Don't worry about it. Many many flashlights have battery rattle. There has to be a bit of tolerance to different brands of batteries that can have slightly different diameters. It will not hurt anything and if it really annoys you just do what others have said and wrap a piece of tape/paper/whatever around the battery.
All multi cell flashlights* will rattle, unless the battery tube is so tight that you can't get the cells back out, or there is so much pring pressure that the cells are crushing each other.
Even my Inova X03 with its super tight body will rattle a bit.
* where the batteries are touching end to end in series