Same issue here and its been driving me f'ing nuts. I have 2 of the GX502's and both have this issue to different degrees of severity.

I have opened both of them up and tried stuffing padding in different spots to stop the rattles and nothing works.

For reference, I also have 2 GU502's (same frame, lower specs) and neither of them have this issue and their speakers sound fantastic.

Opened them up and everything is the same as far as frame, motherboard, speakers, etc. Ive even tried swapping the speakers from the GU502 to the GX502 and it STILL has the problem.

I'm an Engineer & dug a little deeper to try to figure this out. Comparing both the GU502 & GX502 speakers there is one major distinct difference. The GX502 has a f' ton more bass then the GU502. The bass @ 40-90hz is when all the rattles & distortion kick in. The GU502 doesn't force the speakers to go this low. For this reason, I feel that our problem is simply a audio software issue at the simplist, and audio chip hardware issue on the motherboard at the worst case scenario.

Also another issue thats driving me nuts that I guarantee you have but just might not know yet: Go into Aura Creator to set up individually backlit keys. Once you activate it, the right side Shift & Enter keys only half-illuminate. There is no way to fix this.

I have a pretty big following on twitter & have put Asus ROG on blast. They actually replied within an hour to start trying to help. Weeks later I have finally been given a private beta firmware update that the engineers made for me to see if it fixed the issue. STILL NOT WORKING.

I have little faith that either of these issues will ever be fixed and have had to resort to NOT using individual backlit keys (F'ing sucks major advertised feature that doesn't work) & completely turning off the built in speakers & using headphones.

Asus you kick ass with hardware but god damn you guys are terrible software designers.