The past couple of weekends, I’ve been hiking and have been startled by a deep booming sound. Very low frequency with a lot of energy – you can feel it as much as hear it. It consists of repeated thumps, becoming more frequent over the course of a couple of seconds as if someone had dropped a basketball from a height onto a sheet of plywood.
My theories are that it’s either a large bird like a turkey or a grouse taking off (or doing some kind of territorial display). Anyone know for sure?
Not close enough to be the Jersey Devil, I suppose?
Wrong season for grouse territorial displays, tho IIRC they do have fall territories which they
stake out. Typically they “drum” on some nearby log, tho there are other gallinaceous birds
who have air sacs which can make such a noise.
Not in Massachusetts. 
I’m guessing you’re hearing a ruffed grouse drumming.
They do drum in the fall. Cite, which link requires registration, but the relevant portion says:
If you wanna see it up close and personal, John James Audubon sez you can pound on an inflated bullock’s bladder with a stick, thus booming back at the bird, and, its territorial sense offended, it will fly right up to you.
No idea where you’d obtain a bullock’s bladder, sorry. 
The ruffed grouse .wav sounds pretty close (not as much bass as the real thing, but the tempo is right). I guess I’ll check out my local sporting goods store and see if they have any bullock’s bladders in stock.