Weird knocking sound from engine when hitting 110-120 on 4th gear.

I’ve ridden 35,000kms so far and lately I’ve noticed whenever I’m on open roads like the Palm Beach road in Navi Mumbai, when hitting speeds of 110-120+ on 4th gear, I start hearing weird knocking sound from the engine. Sounds scary almost as if the engine is gonna explode or something under stress. What could this be?