Unexplained sound when accelerating. 2013 Honda Civic. Video in description

Question

Video of the sound: https://youtu.be/Z-_1iax3gsg

My Honda Civic is making a metallic grinding/buzzing noise when I accelerate.

Some notes about the sound:

• The sound mostly occurs when I'm lightly accelerating below 2K RPMs and past that it disappears until the car shifts up a gear which drops the RPMs back below 2K and the sound comes back.
• When the car is in park or neutral the sound doesn't happen when lightly accelerating, but if I quickly tap the accelerator to the floor I can hear the grinding sound for a split second before the engine responds to the pedal and revs up.
• The sound is definitely coming from underneath the car/engine and I can hear it more clearly when I drive close to a wall or other cars which bounce the sound back to me.

I've taken the car to a local mechanic shop and they removed the front exhaust shielding underneath the engine bay thinking that it had been damaged and was vibrating when I accelerated. That "fix" made no difference so I brought the car back a week later and after an hour of examination, they told me that none of the mechanics at the shop could figure out where the sound was coming from. They suggested trying to fill up with premium gas next time to see if it was some kind of engine pinging, but after a week of driving on premium, the sound is still the same.

Does anyone have an idea of what might be causing this sound?

Answer 1

UPDATE: I managed to diagnose and fix the problem myself after being turned away by yet another mechanic. The problem ended up being a loose serpentine belt tensioner rattling when the engine was under load. I removed the bolt labeled B in the attached picture and cleaned it up before reinstalling it and tightening the hell out of it. Sound gone :)