Your PC case is what I meant but after reading your initial post again, you meant "shaking" the drives, perhaps by your own hands, not the PC.
I don't know exactly what they use to manufacture LG drives, but it seems most, if not all, optical drive and media manufacturers use outdated and inferior production techniques and equipment. I'm not sure how much such factors can affect product quality. At least from what I have seen with my own eyes, it didn't look so good. Factories in Japan should have at least a little better quality control with high-end equipment and cleaner air, etc. Like many others including Ricoh, Pioneer, Philips, etc., LG has also moved most of their production sites to China in the recent years. Hitachi-made GSA-4163B looks a little different and they are made in Malaysia according to Koba who's in Tokyo. But they seem to cost 2 times as much as LG-made GSA-4163B made in China. 2x as much and a little better quality control and a little better heat dissipation. Matsushita's Panasonic DVD-RAM writer drives must be a little better in my opinion in terms of production quality control, but they cost far more than LG DVD-RAM writer drives even when their features are inferior to LG.