Well, so far no luck. I pulled the entire back seat out. The only reward I got was 0.75 and a few M&Ms.

I pushed/pulled/shook everything I saw but it was solid - which I guess is a good thing. So, I'm going back underneath again - tomorrow. Will put it on jack stands this time so both sides are off the ground at the same time.

Again, thanks for the direction and I'll post anything I find - hopefully, the solution

 

Before you put the seat back in...

The Nissan service manager told me that, in order to reproduce my sound, they had to drive the car with some sitting in the back, with the seat out!

Didn't sound safe, but he said it was the only way they could hear it...

Eph

 

So far nothing new to report. I've noticed that the noise seems to be in sync when the car engages/disengages EV mode, which makes me think it's in the HV unit, like one of the relays. But, the "clunk" can definitely be felt and not only heard so I doubt a relay would cause that. I thought maybe the transaxle but that's in the front of the car. Either way, it doesn't affect the driveability.

Mechanically speaking, there's not a whole lot going on in the rear of the car since it's front wheel drive. Plus, I want to believe that if it's anything electrical there would likely be a code dumped causing the MIL/Check Engine light to be lit. If this is the case, then it's a mechanical-only issue.

Will keep on this until I at least figure where the sound is coming from.

 

I know it's a different car, but just in case your problem is similar to this one...
there was a thread concerning "clunk in the trunk" on a Civic hybrid that was interesting:

http://www.cleanmpg.com/forums/showthread.php?t=30741

Good luck finding the source of the "clunk".