Strange muffled click sound seemingly from rear center when transitioning between electric and gas

Hey guys. Relatively new owner (3 months) of a 2010 Prius Trim 5 w/solar and loving it for the most part. This post will have a lot of info in it to help narrow things down and I would very much appreciate if someone could offer some guidance.

Mileage: 147,000
Hybrid Battery: Original (69% condition according to Dr. Prius App and holding a charge similar to a 30,000 mile Gen 3 Prius I regularly drove in the past)
Battery Fan: Recently cleaned
Front and rear seats: Secured into place properly
12V Battery: Tested and working properly. Under 6 months old.
Transmission and Engine: Original
Oil changes: Consistent and under 5K miles. Full Synthetic.
Under the hood fluids: At proper levels and good condition.

Here's the issue:

When I come to a complete stop somewhere, the gas engine naturally shuts off. When I eventually start to accelerate, at the exact point on the monitor when I transition from the first bar (EV) to the second bar (ECO) and thus the engine kicks on to provide more power to the wheels, I hear a faint but audible singular click coming from what sounds like the center of my vehicle slightly behind me (from the driver position). This happens every single time, and doesn't seem to affect anything else. It has happened for the past 7K miles (Since I purchased the vehicle), and has not gone away.

The only thing I can think of it sounding like is when you click the top of a large plastic storage container together, but quieter and more muffled (as it doesn't seem to be coming from the interior of the vehicle.) A storage bin that looks like this:

I hope you understand what I mean by this sound.

This is the only noticeable symptom. The car still shifts fine between reverse, neutral, drive, and B. It doesn't slip or anything like that, and it has an adequate amount of power. There is no sort of grinding, or noticeable extra hesitation when the engine kicks in.

Any ideas as to what this is?

 

I appreciate your reply.

It is not the door locks. It is at a specific point when electric transitions to gas on the display. Not even at a specific speed. Related in some way to the drivetrain I think.

 

This is definitely possible. Thanks for the reply.

Is those a common point of failure? Is it actually doing any harm, or just a slight annoyance? Is it difficult to fix?