The rattling you're hearing could be valve noise. I also have an '86 740GLE (albeit with over 4 times your car's mileage), and sometimes under load I hear a similar noise. When I run premium fuel, the noise goes away. Try a tank of 93 octane and see if the sound changes. If it is a valve, it's probably just carbon buildup over, as you said, years of city driving. Sometimes a good fuel treatment will help that problem. I'm also wondering if my car would benefit from a new knock sensor for that very reason. Maybe someone else can shed light on that for BOTH of us.

Once you iron out your initial idiosyncracies, you'll have years of pleasure from the car.

Good luck.