Reprogram them to actually make farting noises ... suddenly I wish I was a world class hacker....
Maybe you are not aware but Teslas have had a downloadable program for a couple of years now called Tesla emissions mode with a whoopie cushion icon which does exactly that and you can position the sound for whichever seat you want etc etc. Elon Musk has a sense of humour in a certain weird way. No other car maker has this AFAIK. As Casey Stengel said you can look it up.
LOL! That would be hilarious to watch a Tesla owner as his car farted with every gear shift! It would be even funnier to wire in a can of stink bomb spray.