Erik, I guess I'll reach into that can of worms. . . . (and state right from the start, that this is NOT meant to be argumentive!)
While I don't have nearly as much time behind a .45-70 as most everyone else here, has, I've got quite a bit of time behind the trigger of a lot of rifles/pistols/shotguns/and Class III arms. (35+ years behind the trigger.)
Whether the noise is from the 'ports' or just because of the short barrel, that, to me, is the question.
My Guide Gun is LOUD.
On a hog hunt (March 31), I fired that Guide Gun one time. (Handload using 40 gr. of RL-7 and 405 gr WLNGC hardcast bullet.)
Granted, I didn't have hearing protection (neither did my hunting partners), but, that .45-70 was MUCH louder than one guy's Ruger #1 in '06, and he was standing (literally) 2 feet to the right of me when he shot!
One guy asked, "Who's got the freakin' elephant gun?" ;D
Now. . . the question is (since I was behind the trigger), was the 'extra' or 'excessive' noise from the ports, the short barrel, the cartridge (load), or all of these??
My personal thoughts are the ports and the (lack of) barrel length.