Why wire filament shakes inside a light bulb? My light bulb cast a clear light toward adjacent wall and I can clearly see, that wire filament inside bulb shakes, though lamp itself does not moves even an inch
What is causing this? Is it a permanent effect, happening all the time? Or is it a syndrome of near end of life of this current light bulb?
 A: This really depends on the design of the bulb in question. High-power or hard use bulbs have filaments under some tension and strong supports. Other (mainly decorative) bulbs have very wispy support wires and straight, loose filaments. In this case, the wire can be very sensitive to mechanical vibration through the floor. The bulb and lamp ( a bulb will be mechanically the same as the lamp when screwed into the socket ) have mass thousands of times heaver than the filament, and the resonant frequency will be much lower. 
A: Assuming that the bulb is powered by some variation of alternating current, and that there is a static magnetic field present in the environment, the vibration could be caused by the interaction of the fixed external magnetic field, and the alternating magnetic field caused by the AC flow through the filament.  Bulb filaments are often in the form of a tightly coiled helix, to increase the surface area and allow for thermal expansion.  This can complicate the AC field generated...
A: *

*Is the shaking immediate upon turning the bulb on or after a while? This could mean heating causes

*Is this the only bulb doing this or is it same with other (of same type)? This could mean bulb towards end of its life or bulbs of this type do that due to construction of filaments

*Is this the only bulb doing this or is it same with any other? This could mean sth else might affect the bulbs, like motion or magnetic fields

*Does this happen on this bulb on one place or in other places as well? This could mean the specific place interfers with the bulb

*Does this happen on this bulb on a specific time/day? This could mean sth interferes on specific times, sth enviromental, like TV or cable-voltages
Hope the above provided a procedure to assess your light bulbs :)
A: It's quite common for ac-powered electrical devices which are designed without moving parts to "hum," usually with an audible frequency of 60 Hz, 120 Hz, 180 Hz, or other 60 Hz harmonics.  (In other parts of the world, where the power distribution system frequency is 50 Hz, you'd obviously get harmonics of that frequency instead.) For instance, when I'm baking or cooking, I can hear when the heating elements in my electric stove cycle on and off. Audible sounds are produced by mechanical oscillations of some object in contact with the air, so a vibrating resistive load is not uncommon at all. I suppose the motion must be magnetically actuated, but I've never really thought about details.
Different bulbs will have different filament mass densities and different lengths of filament between the supports, and so like different guitar strings they'll have different mechanical resonance frequencies. I'd imagine that vibration would make the filament more likely to break sooner, and so a good bulb (in the US) would be designed to be quiet at the 60 Hz harmonics. A bulb designed for a 50 Hz market (note that China, which does lots of manufacturing for the US, has 50 Hz power) might vibrate more than the designer intended when powered at 60 Hz. And of course a cheap light bulb might have been put together with no consideration of this effect at all. (You probably know the joke: "I'm returning these light bulbs, they don't seem to last very long at all." "They can't be defective, sir, they're our best sellers!")
If you had strobe lamp or camera with variable frequency you might be able to determine whether its 60 Hz harmonic oscillation or something else. I think that fluorescent coatings have advanced enough that you can't just use a CFL bulb as a powerline-frequency strobe. My DSLR camera has an option called "flicker reduction" that can be set to 50 Hz or 60 Hz; if you have the same thing you might try to video the effect on both settings and see if one or the other "freezes" the vibration.
