Lama Ole’s answer:
First, try to find out whether the source is external or internal. If the source is external—like a cemetery across the way, a slaughterhouse, or a corner with junkies or drunks—then place a mirror in such a way that it sends the bad vibrations back. Octagonal mirrors are usually used for this.
If the source is internal, then you can check whether something actually happened there that caused a lot of suffering. Did someone live there completely alone and devastated for thirty years? Was someone killed there? Did people beat each other up in there? You can try to find out what happened in the past.
On a general worldly level, it is smart to paint the walls or hang new wallpaper—with lighter colors if possible. You should also air out the room often, or you can burn something that drives away the negative energies. There is a resin from cherry trees that works well for this. I think the Catholics even use it in their incense. That is very good!
You can also hang Buddhist protectors of the five colors in the room. They make sure that nothing disturbing comes in again. Or you bring a girlfriend home with you and have lots of joy in that room; that helps, too.