This has been going on since about 1k miles on the car (at 2300 now). When I'm slowing down (foot off the gas) and not using the brakes sometimes the noise the engine makes goes from the regular whine that I've always associated with manuals slowing down to a kind of grindy/rattlycanny noise. This goes away when I disengage the clutch. The noise is completely random, though I've only noticed it in first and second gear. I'd mention it to the dealership, but I completely have no idea how to replicate it, sometimes I'll drive a full week without hearing it, but today I heard it again. Any ideas? Also, I know I have a lot of random questions but this is my first manual car, so I pay attention to everything and baby it as much as possible. Hell, half the time I drive around with the stereo off cause I love listening to the engine (plus I'm paranoid).
It could be just one of the subaru gremlins. They all make some weird noises sometimes. My 06 makes some strange decel noises sometimes but, like you said too, only randomly. I assume you have an 06 as well with that few miles on it. Do you have an aftermarket (read "larger than stock") exhaust that could be rattling around when your transmission is under load during decel? Any more info that you have would be helpful.
pretty sure its normal. It's not unheard of, and I don't believe anyone has really managed to 'fix' it.
My car does that sound, but only because of the exhaust. I can even see the boost gauge needle flutter when the rpms start to come down and I am slowing down.
Its the center diff making the noise and its no big deal. Check your tire pressures. Too much of a difference from front to rear can make it worse.