Tubes for the EGR system (exhaust gas recirculation) can break, causing an exhaust leak and noise under the hood.Google+
Car Problem Reports
Ford F-150 EGR system tubes can break and cause leak/noise
Ford F-150 Problem
Exhaust & Emissions Affected: 5.8L V8
Average Mileage: 95,000 mi (50,000 mi - 140,000 mi)
Visitor, 1995 Ford F-150, 50,000 mi
Check valve rusted causing excessive noise, has not been fixed. I was quoted $200 to replace valve.