We  have had noise testing done at our Range.

Highest decibels recorded at one mile from the range came from handguns. Even higher than the noise from Magnum rifles.

Lowest noise (well below the 65 decimal noise allowance) came from rimfire rifles and Schuetzen single shot rifles.

The only noise recorded above the noise allowance came from a Farm tractor that went by.

Southhampton PA club gallery match has a sub sonic CF class. Way they determine Sub sonic is no "crack" when the bullet travels down range.

looking at my load book I use 6 grs of Unique and my regular outdoor 300 gr bullet in the 38/55, 

32/40 I found a good light bullet for indoors only 125  grs that stays sub sonic with 4.5 of Unique. It's the one I prefer

Long barrel helps a lot, mine are 28 inches. Either one about the same noise as a standard velocity .22 Lr. I found offhand performance about the same as standard .22's score wise.

You may want to look into baffles either side of your firing point 6 feet or so will cut noise outside your property considerably. Does not take much to make a difference. Just reduce the muzzle blast some. Sub sonic takes care of the bullets noise down range.

Caution is light charges of fast pistol powder be real carefull you don't double charge. I made a drop check  gauge. Simple dowel with mark indicating the proper powder charge. Double check all loads before seating the bullet. Double of fast powder will do a lot of damage.

Boats

It is indoors 50 foot ASSRA target, Conestoga Club Southampton PA it's listed in our match section

I don't shoot it often 300 + miles north of me wintertime weather gets in the way.

Pennsylvania used to have another good indoor match in Lebanon PA basement of the Masonic Lodge. Club closed down though.

Gallery CF is a lot of fun and can produce some real good scores.  Southampton runs a Military rifle gallery class too. Lot of Trapdoors and others.  So many classes I had to cut back to shooting one CPA. CF Scope CF Iron then switch the barrel for my RF Barrel then Scope and Iron again. Between Re-Entry Single Entry and Agg legs lot of shooting in one day.

Wish we had that match in Virginia Only indoors I shoot close is 10 M Air Rifle on a Pistol range.

Boats

Me and my friends shoot various rifles at 1350 fps or so, with different powders. When i step away to, well, step away, the rifles sound pretty much like a loud fast 22 LR. 
If i had a neighbor like you who wanted to shoot in a place like you describe, i would like to know that it is done safely and at hours that do not interfere with my nap time. Talk to your neighbors, assure them that you are not crazed and blood thirsty, work out a shooting schedule, test a few quiet loads, and make sure you have a back stop.
I'd rather drive to the shooting range than alienate neighbors. 

Long barrels are quieter. Fast loads are louder. 

Thanks everyone for the replies. I had no ideas cartridges like the 38-55 and 32-40 could be loaded down to those sorts of noise-levels. It looks like I am going to have to make that leap. I am not too worried about the neighbors, but will definitely have a chat with them first. I do occasionally hear a string of shots coming from their way, so I am sure they will be understanding. 

Good idea on the baffles. I think I could make use of some natural baffles in the form of trees and some cat-tails if I were to extend my little range a bit.

Thanks again,

oneshot_geo

The drop ceiling type panels are designed to absorb noise and be cheap and light and should make good light weight baffles, portable and store them in the garage if there is any room left.  Not waterproof.

On a lighter and somewhat illegal note, a Texas game warden caught a midnite road hunter headlighting and killing a spike buck with a .22 with a doggie lamb nipple fastened to the muzzle with a small hose clamp, no discernible noise at 50 yards.  I think that's called a silencer.  Its now legal to hunt feral hogs in Texas with a silencer 24/7-365.  Just have to deal with the feds. 
James

Lot of silencers out there now, builders offer a package deal with assistance in government paperwork . Couple of our club small bore shooters have silencer equipped rim fires.  I have thought about it for my basement 10 M range, air rifle does fine though no need . If I could shoot 50 yards outdoors would go the legal silencer route for sure.

Boats