T-SB-0010-12 Loud engine knock or rattle noise.

If you can stay out of panic mode and think while the ICE tries to burst through the hood (alien style), then simply apply (and hold 5-10 sec) gentle pressure on the gas pedal to increase RPM. This worked very well for me during my last "engine knock or rattle" episode in my Prius v wagon and I think it would work for any of the Prius line.
Edit: link to post re last episode -> Violent Shaking on Startup | PriusChat

 

This SSC does not appear on my VIN. Does it on any of yours?

 

No hits on our three Prius VIN numbers with the SSC.

We have not observed the rough start in the 13' PIP, but it has not been a full year yet. I would estimate that I experience the rough start on average, once per year. It startles me but does not cause any serious concern as I feel I understand the problem and now I know just to increase RPMs.

 

Yes, that is how I broke the cycle when it occurred on my 2012 PIP too.
Seems kind of natural as a solution...

 

I have a suggestion for you. If it does not help it certainly will not hurt anything. Buy a container of Chevron Techron and add it to your gas tank. I have a 2011 regular Prius. One morning a couple of years ago when I first started it in the morning there was a loud knocking noise for a couple of minutes. I put Chevron Techron in the tank about every 5 or 6 months and no more knocking. I put some in yesterday and it makes the car run better. Just follow directions as to when to add it (fuel level). I used it in other cars I have owned. Great stuff.