condensation in the exhaust pipe. When you drive your car the exhaust gets hot as it cools the air condensates and collects in the pipes. If you drive only short distances you might let the exhaust get hot, but not evaporate all the water that was already in the pipes. Try some longer drives to get the pipes hot for a while (30 min at least) then see what it looks like. Being in DE I would bet it's still pretty cool at night too so that will make it worse.
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