My best guess would be something like "When she's with her cat, she has a softer, gentler voice. That's one of her little secrets I know." But "secret truck" is way cooler.
I'm guessing that 'truck' is a typo of 'trick' considering the one letter difference and that the 'i' key is next to the 'u' key on a qwerty keyboard... which makes that last bit 'secret trick'.
To truck with someone is a ye olde englishe way of saying to trade with or make a deal. That said, I prefer Miku having a giant vehicle hidden away somewhere.