I am thinking of two clues:

1) It is accleration-related. Since when I let go the gas it goes away.
What parts are engaged when I step on gas pedal and then disengaged
when gas pedal released?

-drive shafts?
-gears (the car is automatic)?
-CV boots were replaced not too long ago, at that time, the CV joints
were okay. CV boots still fine, not leaking oil
-brake pads, rotors replaced
-engine mount?
-cylinder/ spark plug not firing?

2) It has to do with heat.
Normally, it happens when the car has been running for a while. After I let
the car sit for a few hours or overnight, the problem went away

-the problematic part expands and "slips" against another part
-it sounds and feels like cog wheels sliding against each other abnormally