The pair, who were pronounced dead at the scene, were discovered in Fox Crescent, Chelmsford, Essex, just after 11:00 GMT.
Essex Police said the deaths were being treated as unexplained and a cordon had been put in place due to a "potential noxious substance inside the car".
The scene was declared safe after checks, a police spokesman said.
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A man and a woman thought to be aged in their 20s have been found dead in a car on a residential estate.