Flywheel Chatter
 

Completely stock car. Sounds like flywheel chatter goes away with pressing clutch. Sounds about the same as my 08 350z. Is this something I should go to the dealer for I always accepted that sound as normal in my Z days.
Edit: You can only hear it when idling.

It's normal. I've heard it since I bought mine last year with 4 miles on the odo.

It's actually gearbox chatter. Stuff rattles a bit at low rpm because the engine isn't actually rotating smoothly. Perfectly normal. Gets more intense with an even lighter flywheel.

Not uncommon with most rear wheel drive cars really. The big difference that a makes it so much more noticeable with these is that not only is the drivetrain just a few inches from your knees the only separation you have is metal barely thicker then a pop can and some cloth with flocking pretending to be carpeting. Hearing weird noises is just the price you pay for reduced weight.

Only reason I mentioned the chatter, is that, I haven't heard it in the first two years of ownership.

Then it's something else. If it's a new sound, definitely get some qualified eyes/ears on it.

Can you feel anything new in your clutch pedal? Throwout bearing?

32k miles and my throwout bearing has gotten louder as well.

Very typical in cars with little sound deadening on the economy side of the price range, don't worry about it until the layman can hear the noise. Exedy who I believe may make the OEM clutch assembly is notorious for loud throwout bearings and rattling noises in their clutch assemblies.

It's a Subaru thing, hundreds of thousands of hits if you search for Subaru and transmission/exedy/clutch/rattle/noise/grinding what have you.

Wow, I had no idea it was that common. Meh, I love having an easy-to-drop tranny again.

By completely stock, do you also mean stock fluids (specifically transmission oil)?

i use 85w140 it is chattering
is use 75w90 it is chattering

any good idea then ignore it !!until the input shaft worn out

The reason I asked is because the owner's manual does specifically say in the vehicle specifications section that "Using a manual transmission gear oil other than “MG Gear Oil special II” may cause occurrences of rattling noises while idling and poor fuel consumption." I've no idea how true that is or what they've put in the OEM oil to stop this rattling/chattering though.

Yes, the transmission/diff fluid are still orginal

There should be no issues using a correct rated gear oil, the manual reads the following:

Section 6-1 p.416 of BRZ Owner's Manual

Yeah, there won't be any longevity issues. I'm just pointing out the fact that Subaru does actually state (on that same page) that using a different oil can cause rattling.

(I'm using Motul, by the way; I'm not trying to push the OEM fluid. I do also hear the rattling.)