I have had the engine guard on my '15 XG750 and NO, it did not rattle. The point that annoyed me a little, is that I try to avoid chrome on my bike and have even covered some chrome and painted some of the aluminum rivets black. Anyway, the engine guard uses motor mount bolts on the bottom and while the original bolts are black, the longer bolts that the engine guard uses were chrome. I mean I could see that on the chrome engine guard, but on the black???. I replaced bolts with black bolts and much prefer the look.
http://www.hdstreetforums.com/forum/attachment.php?attachmentid=12266&thumb=1
Don't know if dealer would offer to do it with different bolts to start with. I think mine cost me a about $20 to change the lower and the upper bolts. (although upper bolts are hidden under the radiator shroud)
Also, have taken off my engine guards as I don't see how they would help much anyway. (Also have ordered front controls and I know my foot wouldn't fit with controls and guard).