"I took another video of the engine bay while running, and one of he exhaust. In the exhaust I can hear the distinct puffs as the engine breathes. Not something I noticed in my 300D. Normal?"

Sounds just like mine...kind of like an old motor boat..bloop, bloop, bloop
at idle.

Google "350sdl drive belt tensioner" and you'll see what we're talking about. You lean on it with the hammer handle to see if it gets quite. If so replace the shock.

Google 350sdl idler pulley to see the tensioner pulley wheel.

Belts can make odd noises too. The photo below shows the belt route (disregard hand drawn circle in photo)

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1986 300SDL 440,xxx