2012PriusC, Revving noise when releasing the accelerator

Hey everyone! I've had my 2012 PriusC for about a year now and I'm loving it. As of a week ago the car has started doing this revving thing. I notice that the engine seems louder when I'm accelerating and it seems like I'm getting 30-40mpg instead of the usual 40-50 in cities (but it might just be me).
What worries me is that when I'm driving and I push on the pedal to accelerate, after I release the pedal there is a very loud whirring sound of the engine. It sounds like a MuscleCar or Racecar that is revving it's engine but not going anywhere. This sound happens if I accelerate lightly or severely and the revving sound is louder the harder I accelerate. I notice that if I accelerate and then ease off of the gas slowly, I don't hear the revving sound.
I notice that this seems to happen randomly, as in I start the car and it happens from the start, or I start the car and it doesn't happen at all for that drive.
I took my car into the nearest Toyota place (where I bought it) since it's only a year old but I couldn't "reproduce" the sound for them so they said they couldn't help me unless one of their mechanics (who was in the car as I would drive it) heard the supposed sound. So I'm just asking here since I've seen some posts here about issues with revving engines for the PriusC but none that seem exactly like my problem.
Thanks and sorry if this is in the wrong area. I'm just hoping to get some opinions before I have to deal with the Toyota people again.

 

I checked the vent in the back seat and there was a towel (which I keep on the back seat floor) partially blocking it. I moved the towel and it hasn't acted up driving around today. Hopefully that solved the issue! Thank you so much!