Fresh new data set, guys.
I’ve replaced the lower torque rod, and pretty much as George suggested it was not the root cause.
Still, it was newer/stiffer than the old one and the occurrence of sound changed.
I’ve ran it through the “engine braking” routine - it was similar, while less pronounced sound, but now it appeared at idle, so I could open the hood and explore around.
First of all, it definitely goes from the flywheel / transmission area, now it sounds more like gear droning when idling. It is not from the front, where starter and alternators are, it is located between the engine/transmission and firewall and appears to come from the bell housing, but sure enough it is hard to pinpoint in this tight spot.
Tomorrow morning I’m driving this Sentra to the dealer for diagnostics, crossing my fingers it is indeed transmission as it is still under power-train warranty. Base warranty lapsed at this point.
Sounds like you are on the diagnostic path. One thing to consider, maybe it has nothing to do with the engine/trans/suspension brakes. One time I found a golf ball rattling around in trunk’s wheel well area, noise only happened during braking. Another time I noticed only whenever I drove up a certain inclined seldom used driveway there was a rattling sound under the car, seeming to come form the transmission area. That one turned up to be a weed that was tall enough to scrap the underside of the car as I drove up the incline … lol …
Good idea to remove everything from the passenger compartment, trunk, spare tire, jack, it’s possible the noise will go away.
It’s already down to engine/transmission, so brakes and suspension were eliminated, same as exhaust.
I’ve made a little but of googling on “nissan sentra CVT problems” as well as reviewed www.carcomplaints.com and it looks like 3rd generation of CVT in Nissan is less reliable than 2nd
I want to hope for the best, but my odds getting slim 
an update…
dealer replaced heat-shield and a flange up on exhaust path, next to catalyst… at my expense… but I was willing to pay to make sure it is done “right”…
at that point, they finally were able to hear what I was fearing the most: CVT is droning, even at idle, so I’m getting a warranty unit replacement this upcoming Monday
Good for you for sticking with it. Presuming this will solve the rattling flapping, your persistence with the dealership has paid off in spades. It’s good that Nissan stands behind its products like that. I had a good experience at a Nissan parts dept the other day, so Nissan is on my + list at the moment.
This is 5th Nissan actually out of 7 we had been having in our family 
Still, while brand itself is good in my view, experience varies with dealership a lot.
I’m lucky enough to have one dealer in 2 miles from my office and another one in 2 miles from my home. First ones are complete jerks (nickel&dime you on maintenance, totally unrealistic on sales side), while second one worked fantastic for us both on purchasing side (3 of cars in our family bought there) and so far on maintenance. Sourced pars for my old Pathfinder there too. Guess where I brought this Sentra for repair
FINALS:
picked up the car today after CVT replaced under warranty
no rattling/droning sound anymore, transmission is behaving like it used to when car was new: smooth deceleration, fast switch back to acceleration (no sag we experienced lately)
I did not talk to the mechanic as I came too late from the office, was picking it up from the cashier.
papers show “remanufactured unit” plus normal items like fluids and single-use-fasteners and clamps.
checked under the car: CVT unit shines new aluminum and bottom pan shines new paint and bolts… not sure I’m lucky to get new unit or maybe it was “deeply remanufactured”… who knows… still I’m very much concerned how long this “new?” unit will last
they also replaced heat-shields and to do that they decided to lower entire exhaust down, replacing spring-loaded flange in the same time
CASE CLOSED
Change the fluid at least every 30K miles and drive gently…that’s all you can do if you keep the car.
the end of your reply pretty much summarizes what I think: IF I KEEP THIS CAR…
my wife wants to drive it further, I’m suggesting to sell it in a couple of years maximum
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