Sorry for the delayed update..! Finally sorted the thing out!
It was funny that none of the mechs I went to could find the simple issue there, until I figured it out myself. (Thank you Sampath Gunasekera!, saved by your technique)
Strangely it turned out to be the right hood hinge giving some abnormal sound at specific speads. Oiled both the hinges and the sound was all gone.
Thanks everyone !