The closest I've gotten to hearing absolute silence was in the Bose anechoic chamber (I had friends who worked there). Minus ambient noise what I heard was my own heartbeat. I of course found the experience a bit disorienting. I would like to do it again, but those friends have moved on.

Some other interesting test rooms there included one in which an entire living room was attached to the ceiling - so A/B tests could be made without the possibility of any furniture having been moved (and so messing up the test), and a 'loud' chamber fitted with an absurd number of their 'sound cannons' to simulate environments their headsets (avionic and military) would be used in - simulated firing noise from inside a M1A1 was pretty... visceral. As a private pilot who mostly relied on David Clarke headsets (AKA David clamp-heads) their headsets were a bit of a revelation in how not-painful aviation headsets could be (actually as comfortable as a knit cap) and still eliminate the noise. When I went on about them to my friend in the headset dept he mentioned the ear cushions were filled with the same silicone used for breast implants. And now I’m super off-topic...