Did anything happen while driving, "ie" off roading, accident? Just checking if something vibrated or just came loose. Possible speaker problem causing a feedback to stereo. Check wiring and speakers, or have a audio expert check it for you.
RepairPal Certified shops can diagnose and repair your 1998 Toyota 4Runner. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.