On wet streets the noise from the tires is also a lot louder. Another thing we're completely used to. But once you pay attention to this, you realize just how much louder it actually is.
I don't even hear the tires on the little Google Pods they're driving around Mountain View these days, which seems to have little bitty tires like a smart (wise for efficiency), but the drive is quite audible. As an aside, it's oddly futuristic, though; I think they did that on purpose because when it goes by I get confused and think I'm in Star Trek for a minute. It genuinely sounds like background foley in Hollywood future movies, and I'm not sure I really mind it.
I'd laugh if Google put all this money and time into self-driving cars, a big executive-involved project at a behemoth, and in the end the engineers gave it the Star Trek background-whirr noise on purpose.