I get tinnitus now and again, it's not unlike this... Have they actually captured the sound on recording? They all seem to be only describing it, if you can't record it then the noise is probably in your head. Not to say you'd be crazy, but perhaps it's a resonance of some silent frequency hitting a structure in your body and vibrating at an audible frequency within?

Government was experimenting the ELF waves and making people hear things.