[FireFart]

As object, had a rattling noise issue, gone underneath and identified the guilty quite easily, it was inside a squared metal box, and at a certain rmp range it produced the sound
Please help me identify it as i would like to avoid to go to service without a real reason

it's a sturdy, heavy piece of metal, upper part is in some sort of rubber

 

[FozzieBalou]

A few things -

It would help to know what kind of vehicle we're talking about - and what year.

It could also help to know where you found this piece - beyond "in a square metal box". If you cannot describe it, pictures often help.

Finally, this could easily be a piece of some other vehicle that was kicked up on a trip and is not really a piece of your car but of somebody else's car.

 

[FireFart]

Thanks for the quick reply ^-^ i'll be more detailed as soon as the sun will rise again

 

[Kevin]

G'day & Welcome @FireFart
Could you please update your Public Profile; vehicle details at least; thanks. For detail instructions pls refer: Your forum "Public Profile" - how to fill this...

 

[vintage42]

It looks like a bushing mount -- a metal sleeve containing a hollow EPDM rubber core that should be holding something like a suspension part.

 

[FireFart]

Hi Guys,
Yes, i will update the profile in short time, sorry about it. Edit: Done
I have another picture. It is definitely a piece of my car, it was meant to be attached to a internal side of the box you can see in the center of the picture, placed under passenger seat, definitely related to exhaust, the cylindric piece has not pass trough hole

,

 

[vintage42]

The exhaust on my BMW Z3 has a heavy metal and rubber damper part hung off of it that is called a vibration absorber.

Vibration Absorber

This Genuine BMW Vibration Absorber (Mfg#18301435672) fits BMW Z3 1.9 M44 1.9L.

www.ecstuning.com

The OP's loose part from inside a metal box could be a damper mass that came loose from inside the box in his photo, the whole thing being a vibration absorber assembly.
Here is an enhancement of the OP's photo of his exhaust:

 

[FireFart]

Yes it looks like a counterweight, no electronics but it would be reassuring to know exactly its function and if it's safe to leave the car as it is and just consider to have a new fancy paperweight
another pic of it :

 

[vintage42]

The photo shows no hole through the rubber center, and the steel shell is unusually thick and heavy, and is painted.
This rules out that it might be a pressed in rubber mounting bushing. It is mostly likely a damper mass that has come unscrewed and should be easy to put back on.
A vibration absorber would be part of Subaru's effort to reduce noise, vibration and harshness.

 

[FireFart]

it seems legit, nevertheless the piece has no thread, it looks like the "rubber" (epdm should be a plastic alloy of some sort as i assumed by google) has simply ripped off, the piece is full metal, the epdm is just a thin layer on the top

 

[vintage42]

EPDM is synthetic rubber, not a plastic. It is resistant to high temperature, ozone and weathering, and is used for molded parts and sheet roofing.
As for that part, I don't see it on Subaruparts.com.