loud noise when driving

Hello everyone, this seems to be what's going on with my car. I'm part relieved, but not entirely since it could always be something else (and bigger). Hopefully not!! So thank you for all the previous posts
I've been surfing this forum for ideas of what could be happening with my car and I THINK this thread has the most similar details. My 2007 Prius II is starting to sound like a little roadster - when I was driving on the freeway behind a large car I thought it was them, or just hoped we braked and hit the accelerator at the same time thinking it couldn't be my car. Only after that car left did I realize it really was my usually-ninja car making the loud purrs. It cruises quietly and the sound goes away, it's hitting the gas that makes it loud. The mileage is the same, it feels the same, it's just loud now. I'll bring it in and suggest the ideas I've seen here and keep my fingers crossed that all is ok.