Vtec is engaged via solenoid, that could maybe cause a sound like that. And the fact that it happens 100RPM before you "hear" vtec engages, I'd entertain the possibility, that vtec could possibly be engaging before you hear it. That would make me troubleshoot the vtec system.

Also, maybe it is the oil windage sucking up more oil, right before vtec, or a semi blocked passage somewhere, and you're hearing a high pressure oil sound.

I'd consider: oil change, check for metal fillings. Maybe some new grounds if yours look shot, check wires for frayed spots. And then after all that, maybe pull the vtec solenoid, I hear there is a screen that you can clear out.

-PHiZ