Hi Guys!

It has started happening since 2 days. WHENEVER, I put my Machine on some sort of Scanning or say Defragmentation, after few minutes(7-10 mins.), CPU starts sounding "Ambulance Siren"; a long continous, unbreakable siren that totally resembles to  an Ambulance Siren.

Worth to note, this do not happens in the normal course of action(while work as usual on the system) & only while I leave my system Idle ( turning OFF the Monitor) for Defrag or Scanning with say Stinger..!

Is it indicative of some forthcoming Hardware failure, overheating or probably abrupt halt of say CPU Fan?!..Please help!

sounds like an overheat siren. My old COMPUTER would do that only with particular games.

likely means the heatsink is clogged with dust.Ya most likely overheating.  You want to open it up and put something in the fans, including the POWER supply fans so it doesn't move (like a twistie-tie or weedwacker line).  Get a can of compressed air or set your vacuum on the blow setting and blow out all the dust.  Make shure you take the twistie-tie or weedwacker wire out first!Never place a vacuum whithin 2 feet of a PC.....blow setting or not.It creates STATIC right? I thought it was ok to just use the blow.  Guess not...Only compressed air in the case, and not close enough to freeze any components.
You get to vacuum the mess outside the case afterwards.

Maybe download Speedfan and see how hot that puppy's running.
Its overheating. TRY getting software that can warn you if it get to a certain temp, or buy a new fan.