I have this problem with my violin. Whenever i take it out to play, I hear something rolling around in it. It's not anything that's NOT supposed to be in it (e.g a penny). It's more of a slim wooden cylinder. I can't seem to remove it. What is it? How do I take it out? Is that piece vital to the playing of a violin?
Be sure not to wait too long. Reattaching a sound post, or at the worst, refitting a new one is a pretty standard operation.
It is important for the sound quality and it also carries quite a bit of tension originating from the strings and the bridge. While some of it is also taken by the bass beam (running in parallel to the strings under the cover) in instruments these days (including basically every old instrument that has been refitted for modern string tension and bridge position), the sound post is rather important for taking a load off.
If you are leaving your instrument for too long in this state, the cover will start sinking in, beginning near the bridge, eventually causing a decline in stability and sound quality.
A luthier can fix a sunk cover when its decline has not been too great, but it is quite the process requiring disassembly of the violin, a plaster cast and weeks to month of warmth and pressure and gradual hollowing of the cast.
You can imagine that the cost for that is not near the same order of magnitude as that for replacing a sound post.
So don't wait.
Detension the strings immediately and get to a luthier posthaste. The soundpost provides structural support. If not in place, there's a lot of tension being placed on the face of the violin, and it can collapse, and the seams can also come unglued/split. Putting a soundpost in place takes a few minutes and is usually not even something they'll charge you for.
Source? Playing violin for 26 years.
Sounds to me like the sound post. Normally held in place by friction, it's supposed to act like a pillar between the front and back of the violin, to provide structural support and enhance the sound quality by transmitting vibrations to the back of the instrument. It sounds like its pretty important, and should be a simple matter for a luthier to repair.
As answered already this is the sound post.
The fact that its not where it should can harm your instrument!! The cover might be defformed, the bass-bar might start to unglue and eventually the cover might crack.
Loose/release the strings tension and don't play on it until its fixed. The bridge might fall when you loosen the tension of the strings. This is ok. Just be sure you kep it safe untill you reach a luthier.