Hi, I have the exact same issue with a 2080ti trio x, the issue has been there since I received the card, however only the small fan is affected. Just like you, the noise appears when the fan starts, and even at a very low rpm. I'm thinking of rma'ing the card as the noise is annoying.
So my card is back. They replaced whole cooler sytem. First day card was working fine and quiet. The next day strange noise came back ;(, admittedly it's a little less annoying. That hissing is It's a common noise for a ball bearing-type fans probably ;( . I think TORX fans 2.0 was better.