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Rattle at start of acceleration

My 2011 Forester makes a rattle sound each time I accelerate either uphill or on level ground. The sound is reminiscent of the pinging sound from cars from years ago when we were told to use a higher octane gas. It’s only at the start of the acceleration, not continuous. The sound seems to come from below the car, either under the hood or under the body, not inside the passenger compartment. Most websites say loose heat guards, but this would mean I’d hear the rattle throughout the acceleration, wouldn’t it? Brought It to the dealer twice and they couldn’t hear anything. No surprise. Anyone else hear this sound too? I hear it with my 2013 Legacy also.

I think you have a broken weld on a heat shield for the catalytic converter.

common problem and easy to fix by tightening it with a large band clamp.

Try reaching under the car and shake the tailpipe real hard…up down and sideways.
You won’t break anything, so be forceful.
You will probably hear the same rattle.