The FAA has issued a nonbinding special airworthiness information bulletin (SAIB) urging aircraft operators to replace mufflers at 1,000 hours. It's not a noise abatement move, but rather a safety of flight issue. According to a Wichita State University study, a high percentage of aging mufflers are involved in carbon-monoxide-related incidents. In 92 percent of cases in which a CO leak was traced to a defective muffler, the muffler in question had more than 1,000 hours of service. So the SAIB suggests replacing mufflers at 1,000 hours.