Hi, sounds like a buzzing reed, could have dirt in it or metal touching the edge of the reed frame. Sometimes when they change climates the shrinking or expansion of the wood will cause this. A whole lot of things could be happening but not really a big deal and most problems are fixable, part of owning a harmonium.
I would highly suggest you get the Harmonium Handbook, I carry it here (at the bottom of the page): http://raincitymusic.com/sitar%20tabla%20books.htm
It's a common book and easy to find on the net as well, it'll help you get to know your instrument though.
LarsRain City Music