2011 2.5i Premium - Knock on very low throttle?

Hello Outback hive mind: we have a 2011 2.5i Premium and it has a strange issue that I can repeat over and over again, but it's really subtle... When you first slightly touch the gas pedal, you can hear a faint knock (or two) as the throttle engages and the vehicle starts to accelerate. It has to be the lightest touch on the throttle, and it happens cold/hot/whenever.

What could this be? I haven't been able to find anything online that quite matches what I'm experiencing, so I'm turning to everybody here. CVT issue? Torque converter thing as it engages? Maybe a pre-detonation knock?

It's going to hurt, but I'm planning to rule out pre-detonation when we fill up the tank the next time; I'll put premium fuel in it and see if it still happens. I'm also planning to clean the throttle body this Friday to see if that clears up some weird idling issues.