I've noticed recently that my passenger seat is quite noisy. The highways I drive on for work are bumpy and uneven, and the passenger seat seems to shake a lot. It makes a constant rattling or tapping sound, and it does so at a fast pace. I can confirm that it's the seat, as when I reach over to hold it down with my arm, the noise stops. It only seems to be the backrest of the seat, not the part where you sit on it.
My car is a 2006 Toyota Corolla. Are there any tips for quieting a noisy seat?