Random rattle at higher speeds

Hi!

I have 1998 outback, 112k. I’ve noticed a rattle coming from under the car when the car is jarred at high speeds, such as on a rough freeway or uneven pavement around expansion joints. It’s especially noticeable after I’ve had the air conditioning on—and very loud—as if something was about to fall out from underneath or something is bouncing around loose. No such noises at low speeds or over speedbumps. I’ve had it checked out before, but the mechanic couldn’t get the car to rattle.(Of course) What are your thoughts?