Q:

Strange hum sound while playing open B note on the second string

Four months ago, I bought this Yamaha f310 beginner's guitar. I did not have any problems during practice.
Since yesterday, when I play B note open on the second string, it sounds fine and when I stop it with a finger after a beat or two, some sort of hum noise continues to come from the sound hole. I thought may be the instrument is out-of-tune but when I checked with the tuner, it's fine.
I'm not facing this problem with other strings, I play them open and stop them with a finger, it stops and doesn't produce any hum. And interesting thing is, after playing B note open on the second string and stop with all my fingers holding all the strings, it stops. 
Unable to concentrate during practice when the hum noise continues to come even after stopping with a finger. Can anyone address the problem?

A:

It could be sympathetic vibration, of the bottom E. B is one of the harmonics that sound when an E note is played. Firstly, try muting the bottom E, while playing the B open. 
It may be that there is something near where you play that picks up this vibration. Does it occur wherever you play, or only in a particular position in a room?
Occasionally, a machine head (usually on a well used guitar) will have a 'loose' position. This can produce vibration which can come across as a note. Check there are no other loose parts, like a strap button.
At only four months, the guarantee is still valid. Failing everything else, it need to go back to the supplier.

