My 2002 Avalon XLS was handed down from elderly people who never listened to the radio. After some time of driving it, the speakers begin to crackle and make very loud noises. On the rear subwoofer speaker, I was able to stop the buzzing by wedging a napkin in between the cone and the mount of the speaker. The other interior speakers still make the same noise though. How can I stop this and what could be causing this? The speakers were never put to their limits.