Exhaust noise

If its a spike in a give frequency at a given rpm, drone in my experience, it can be eliminate by using a branch resonator on both tail pipes. They are not in the exhaust gas flow but branched off the tail pipes. You can put an app on your smart phone that will show you a db verses frequency graph and you can see the spike appear. Once you know the frequency you can calculate the dimensions for the resonator.

Yesterday I put them on my 67 GTS. No drone. Check out the thread about suggestions for mufflers without drone. I tried a few including the Dynomax VT that is suppose to stop drone. The VTs reduced it 12-14db in my case but didn't eliminate it.. The branch resonates wiped it out.