Re the buzzing, if it only happens on certain notes, that suggests some sort of volume resonance in the machine. Acoustically, resonances are caused by reflections of the waves inside the volume. The way to fix that is to change the conditions at the boundaries (to make them harder/softer, more rigid/less rigid, etc...) I had a similar problem with my Yamaha P250. Certain notes would cause this horrible ringing sound that just grated on my nerves. I found that simply loosening the screws holding the side that was ringing greatly reduced the problem (presumably it made the boundaries less rigid). Had that not worked I would have tried wedging some thin rubber dam in as shim to reduce the vibration. Now I'm quite happy with the sound. I don't know if this is something that might work with the casio or not, but I thought I'd mention it just in case.

It is not the wire or some other out of box things cause the vibration issue. I put my px 350 on the carpet, it still makes buzzing sound from the left speaker for the same notes.

It is quite related to temperature and humidy. In some rainy and cold weather, the buzzing note will shift downwards or disappear completely, but some higher key start to buzz the right speaker. But in sunny day, the left speaker start to buzz again.

I have returned my PX 350 two times and they finally admit the defect and don't bother any further return/