It's been a couple of nights that this happens. So I get up, as I can't sleep because too loud, even with all the masking stuff: mp3 with falling rain sounds plugged in, fan on, audiobook on. Terrible. It sounds like an higher pitched version of a heart monitor signal of someone who died. Fortunately, this damn thing subsides a lot about 20 minutes after I get up. It makes me think it's something related to blood pressure or vascular problems. Also, the low pitched 'rumble' returned after a couple months vacation. This rumble is like a walk in the park compared to the hiss. It's much easier to mask. I feel better now, but when I wake up with the latter in my ear I feel so pitiful and miserable, as I have never felt before, because of uncertainty and doubt.