I have a Cannondale Synapse SE 105. Whenever I go downhill I tend to shift into combination of top (smallest ring) gears in back and large gear in front. My bike just makes so much amount of noise every time I try to pedal in these combinations (I think the chain is rubbing on front derailleur). Can anyone please help me on how to eliminate this sound?
If you think your chain is rubbing on the front derailleur I think it's safe to say that the noise is coming from the drivetrain.
Front derailleur rub tends to occur when the chain is at the most inboard or outboard positions, either small chaining / largest sprocket or large chaining / smallest sprocket. I would not expect to get rub on the 2 or more of the smallest sprockets, unless the derailleur rub is very bad.
In any case, the first thing to do is to check the front and rear derailleur adjustment. (You may be getting some noise from bad rear indexing.)
Detailed written instructions would be far too long and difficult to follow, but there are many instruction article and videos available on the Web. The best by far I have found are those from Park Tool.
Run through those adjustment procedures and see if that fixes the problem.