Oly, that is indeed interesting. Hard to deny our natural wiring. Spacecraft are actually pretty noisy- creaking and popping as sun and shadow heat and cool the exterior; the clicking and whirring of countless fans, compressors, timers... (Not speaking from personal experience here). I find myself most relaxed with gentle nature sounds in the background.
When I lived in Chicago I was fortunate enough to have access to this great place called Spacetime Tanks. It was owned at the time by friends, and my art hung in the lobby and waiting area, so it felt comfortable and familiar enough that I was able to go into some altered states there without getting myself into a weird situation (important to consider!)
First I went for an hour, and found the experience lacking. So I went again for 90 min and found that things started to get interesting right before the end of the session, when that knock on the tank came (by the way, simply knowing that the knock is coming at any moment is a big grounding factor). I found that I had entered a state of little or no thought, kind of a classic meditative state, but arrived at without trying. Time became quite murky and indistinct. Stepping into the shower, I found that re-entering classic time flow was odd and a bit giddy. Time, what a concept!
Later I went on a number of 3-4 hour floats under decent dosages (400ug acid, 3.5g shrooms) and time got even looper. These experiences were distinctly nonvisual, even the very deep places. Re-emerging from these moments onto the crowded Chicago streets was a little crazy.
And it's true that its not total sensory deprivation: the fan makes a whirring sound, there are sloshing sensations when you move, and you get little itches along the waterline where the salt is drying onto your skin. But nonetheless it gives you access to places that are quite different from being on a sling chair on the back deck.
Incidentally, I found that on the long sessions my arms floated a little funny and caused shoulder pains after a while. Solved this by rigging some elbow weights using coins and socks.