The one sound that humans will never truly hear. True silence would actually make you go crazy and is not recommended whatsoever. The problem is that the quieter the room, the more your ears will be able to hear. They'll adapt and you'll start to hear your own blood flow and maybe even lungs or heart. That isn't something a human should think about for an extended period of time.
To sum it all up, your own sound is what will drive you crazy. Imagine a pulsating headache that keeps on coming back. There'll be the highs and the lows and if you're unlucky, the highs will keep on getting higher.
There is a silent room that is 99.99% sound absorbant. To achieve this level of sound absorbance, it definitely wasn't cheap, $1.5 million to be exact. It is so quite, that no one has been able to last longer than 45 minutes within this room. It might sound like a super easy world record to obtain, but is it worth risking insanity?