Thanks Simon.
Well, I tried already another set of speakers. And I just tried the speakers which are integrated in my flat screen. The same result is there.
Yes, there's a cordless phone just beside the PC speakers. That was always the case with no problem. When I tried the other set of speakers, they were well far from that telephone base. And the problem continued to be there.
In fact, I don't see something else new which was added recently around the PC. (Unless an unknown electric problem somewhere else in my room, maybe).
Thanks again Simon..