I wonder if it has something to do with the cabin condensation drain being above the transmission... lol maybe you're literally hearing ice crushing; a block of ice builds up after your last drive and is between the trans and the body, when you hit the gas hard from the stopped position there's a lot of mounts that flex and probably crush the ice that built up.

I've never heard this sound but im normally parked in my garage over night. My XK was outside for this storm so i'll have to listen for it when I go for a spin later..