avoid tailpipe lengths behind the muffler in the range of 55 to 75 inches, otherwise you have created your own 'church organ pipe' that will resonate

alternatively,
break up these bad lengths with resonators... or use a muffler design with a 'Helmholtz Resonator' section

the noisy resonance depends on the
temperature & pressure of the exhaust gas too
(for geeks: actually the speed of sound for that gas condition)
so the rpm the noise is worst at will change slightly with the weather and with the throttle opening (you will hear this effect going up and down hills at the same rpm)

you can prove this to yourself by adding a two to three foot long section of exhaust pipe temporarily

notice that many imported cars have the muffler at the very end?

the science of this is the same as with
exhaust primary tube length on headers
and with intake manifold runner length

want to be very afraid of this effect?

see the movie
'The Perfect Storm'
where the gigantic "Rogue wave" at sea
is caused by the same resonance effect