Loud engine at start up?

Hello all. New member but been a reader for a few years. I'm hoping you experts have some advice.

I have a 2006 prius I have been driving for 3 years, prior to my ownership it belonged to my mother. It's currently at 217k miles, the majority of which are highway miles, it's crossed this country coast to coast multiple times. It's always been regularly maintained.

A few weeks ago it gave me all the dash lights and a loss of power - dealership confirmed hybrid battery failure. I had a company come out and install a refurbished battery on 6/16. It's ran perfect since the replacement 10 days ago. However today I went out to start it and it's making the most horrible noise as soon as the engine kicks on at idle. I turned it off and restarted a couple times, the sound persists.

No dash lights or warnings are on whatsoever, but the engine sound is deafeningly loud, the video doesn't do justice to how loud it is. Initially when the sound starts it speeds up and slows down rapidly, as though accelerating.

Waiting to hear back from my mechanic, but not even sure I feel safe to drive it down there

 

My husband got home from work and had me back the car up, where it was sitting on the driveway were these:

 

Well the problem has been solved, though I'm none too happy about it. Apparently sometime in the middle of the night, someone climbed under my car and sawed through everything from the catalytic converter back about 3 feet.

 

Yup that's exactly what happened, thanks guys. I didn't even know that was a thing. I'm blown away they would target my 14 year old prius when there is a 2020 passport and a 2018 Fit in the driveway but I guess so. I'm so mad. Does anyone know if those catalytic converter shield things they make are any good? We have cameras going in today, and from now on the dogs sleep in the living room with the windows open, instead of back in our bedroom. I just dont know what else to do. Since they know a prius is parked here, whats to stop them from doing it again?