I noticed when the volume is turned up 80% and higher I thought the speaker was distorting. Turning the volume down to roughly 70% or below prevents it. Thought the speaker might be blown, but then had a hunch it might be the plastic back cover vibrating when the volume is turned up, so I took it off and.. although there seems to be slight distortion it's not as noticeable, put the cover back on and there it is again. Anyone else have this problem?