On the right side of the car under the hood is the radiator overfill bottle it is mounted very flimsey you could rattle it just by hitting it with your hand.
Try this . Get a spong and wedge it in between the bottle nd the AC line that runs under it. This woll hod it without movement.Drive it again under the same condition that make the noise and see if it went away. if it went away then you no what is causing it.