My 86 Toyota truck recently started to drone at 2800 rpm (highway speed) only. The exhaust manifold to cat are oem with 462,000 miles on them. The cart back is an aftermarket Bosal system that is 7 years old with 78,000 miles on it. It's been almost dead quiet in the cab up until the last few months. I don't see any leaks or holes in the system. Question is 1) Can a muffler go bad inside and then start to drone? 2) Could a leak in the exhaust manifold gasket cause a drone? Thanks.