Just had a thought--the hens don't do the loud "gobble" like the toms, so that might be a possibility for you. If they got too loud you could always sell them easily. The do have this kind of semi-loud "RRR-RRR-RRR" call that they do, but mine only do it when they're free-ranging, like they're...
They also get HUGE, very quickly! Definitely hard to hide.
We have a neighbor on one side who's fairly close, and I know he hears our turks, but it's pretty much all country folk out here, and they have a good tolerance of such things. I find the turkeys, even with their outbursts...
YES, they are noisy! If your neighbors are at ALL close, there is no way you can have "secret" turkeys.
They gobble--LOUDLY--at the slightest noise or startling sights, and the more of them you have, the louder the noise. Now, their "regular" sounds they make while just roaming around...