After all the off roading I had done with my NV hub the freewheel started getting a grinding noise that I could only notice while spinning the cranks backwards while I was off the bike. Apparently I got some sand or mud in it over time. I remedied this by blowing lots of WD40 through the gap in the freewheel while spinning it (off the bike).
Once I couldn't hear any more sand grinding around in there I spent maybe 30 minutes cramming grease into the small gap in the freewheel until I was satisfied the whole thing was packed. It spins as if it were new again.
Try cleaning around your sprocket thoroughly with soapy water and a rag first. Make sure your chain tension isn't so tight you could pluck it like a guitar string. If it WAS the freewheel the squeaking would probably get better when you pedal, not worse. Try pedaling it on a stand so you can listen for the source of the squeak. Even a loose spoke can squeak. I doubt its the hubs internals, if it is theres nothing you can do about it anyway.