I was lucky enough to get a ride behind the late Lefty Gardner in his gorgeous P51 and though I then had little aviation experience to go on did not find it excessively loud. I was surprised at how smooth and deep the engine sounded (none of the Merlin crackle) and how regular - constant - the sound was. A loud Fergie tractor is actually a very good description. Integral cloth helmet so not much sound moderarion. It was much more mechanical noise and a lot less exhaust and prop than spectarors get. I recall lot of blower +gear whine and a whole gamut of noise from the mechanical parts which at first sounded a bit alarming as you never get that outisde - as with a car, but drive one stripped down and you hear every tappet, valve, camshaft and piston yammering away. Guns over all that? I'd guess more of the same, a deep mechanical drumming from the breech-blocks and little muzzle noise.
G? Sheesh! Six and then some I reckon. I had done a couple of aerobatic flights before and knew to stave off GLoC but it didn't work with Lefty. 350Kts on a knife edge between his house and garage looking up at the first floor windows (I swear it!) and pulling 3 or 4g was his way of saying "Honey I'm home". Then land on his short, very stony and rough strip, pause at the road to check for cars, taxi across, onto the grass and park by the pool.