I haven't yet been able to witness the RPMs dip when the compressor kicks in (my eyes are usually on the road not the tachometer whenever I hear the rattle), nor have I been able to purposefully cause the rattling sound by switching on the A/C while in idle. Maybe it will help if I turn the thermostat down to LO before I turn the A/C off, wait a while and let the car warm up, and then turn it back on while in idle. I've never heard the rattle while idling at a dead stop though, it only happens around 5 to 35mph. Sometimes its loud, sometimes it is very quiet or barely audible.

The rattling noise appears to be normal
 

I found the thread: http://www.ft86club.com/forums/showthread.php?t=12429

I'm ashamed I didn't find it before posting my own thread, but I did try searching the forums before posting.

Here is a video from that thread that shows the exact same noise that I am experiencing, and it also shows that the tachometer needle DOES drop to about 600rpm and jumps back to 700rpm when the rattle stops.
http://s421.photobucket.com/user/whs..._1331.mp4.html

After reading through all 5 pages of the thread, most people are blaming either the direct injection system, lightweight flywheel, throw-out bearing, gear box, or just harmonic oscillations for the chattering noise.

Furthermore, there is no solution to this noise yet and we probably don't need to be concerned about the noise. As long as we keep telling dealers that we are experiencing this noise, I think the word will get to Subaru/Toyota and they might give us an answer. For the meanwhile, the chattering noise seems to be completely normal, happens to most if not all BRZ/FRS's, and most likely will not damage our cars.

Thanks everybody for your responses.

During engine braking the computer cuts the fuel supply. When just idling (which is what happens when you're coasting in neutral) the computer has to feed fuel to the engine to keep it running. I have my doubts that it really saves you enough fuel to make any significant difference though.

That's not why some of us downshift. A performance car actually handles differently when it's in gear. When coasting out of gear, I don't feel like I have complete control of the car, nor am I or the car ready for unexpected situations that might require immediate maneuvering. Take your car to an empty parking lot and try slinging it around a bit in gear and then while coasting, and you'll feel the difference.

There are also some good arguments regarding the way gears wear that support engine braking. If you're always applying power to gear teeth from the same direction, they can wear unevenly. Engine braking evens out the wear pattern so that the teeth last longer and bite better, or so the argument goes.

I have this as well. Its Engine cutting off fuel. Happens even after my OFT stage 1 reflash.

I agree that the mpg difference between coasting in gear and in neutral is
insignificant. Tools might read about 300mpg for neutral and 9999mpg for in-gear, however, idling in neutral might only consume one gallon per hour. And the time I spent coasting in neutral was not very much, only a few seconds before every stop. So maybe coasting in neutral might cost only $15 to $20 extra in gasoline for an entire year.

That's an interesting point you made about gear wear, I never knew about that. Some people believe that leaving the car in gear while coasting may slightly increase wear on the engine (instead of wearing on brake pads in neutral), which would make coasting in neutral a better choice. But the extra wear on the engine is probably next to nothing.

Thanks for posting. I believe I made a thread asking the same question but it was disregarded. I'm surprised not everyone has noticed this yet as I hear it often, especially when I'm coming to a stop from a downhill and I put it in neutral a bit earlier. Seems like a 10% chance in these situations.

So stop putting it in neutral from 30-40 mph.

When coasting in neutral at any speed you have taken your car and turned it into a big, heavy (relatively),expensive (again relatively) soap box racer!
As soap box racers are not generally know for their handling, speed (other then down hill) or ability to avoid sudden changes in the driving situation or conditions you are usually better off to have the car engaged in the appropriate gear at all times. I know that accelerating quickly or a sudden swerve under power has saved my butt as many, or more times, then hammering on the brakes and if I was coasting in neutral those times I would have been screwed.

Mine does this as well, only when the AC is on.
The rpm get's so low, the daytime running light turn off sometimes. I saw this happening several times in the reflection of the car in front of me.
It's just annoying, as long as it does not stall...