It might be similar to (albiet the oposite of) how putting on a pair of decent active noise cancelling headphones "sound different" (even if you were in an otherwise quiet room) and/or many people report a "head pressure" feeling.
Their brain is used to filtering out the background noise, so removing it causes strange sensations.
I don’t think the head pressure feeling is something the brain automatically adds to silence. I get that feeling from noise-canceling headphones but I don’t get it from a sensory deprivation tank.
Their brain is used to filtering out the background noise, so removing it causes strange sensations.