Hybrid System noise?

Just this week without any warning I started to hear a noise which must be coming from my hybrid system of my vehicle listed above. I heard it before I started the vehicle and hear it sometimes when I come to a stop. There has been no change to the driving of the car, and it does not seem to be related to the engine. When I open the door, I have always heard some click type sounds but this is new. It is more a low-level sound like chuu, chuu type sound and it stops after a few seconds. I took it to my mechanic who agreed and said it is coming from the hybrid system which might be difficult to find and expensive to fix so he said it does not affect the drivability of the car after he took it for a road test that I should just leave it alone and not to worry about it. I live in the city and again it is not mechanical but a low guttural sound that is infrequent but starts when I open the door and sometimes when I start the car for a few seconds and also sometimes when I come to a stop. On the highway there is nothing. I understand this is not a clear description as it is not very loud, but should I find another mechanic to diagnose this and does anyone have any idea what I am hearing and if this will cause a problem at some point. Again, it is a soft sound which seems to be coming from the hybrid system of my 2013 Prius.
Thank you for any advice,
Larry