400SEL, I associated the ticking sound to leaking exhaust at the exhaust header to exhaust pipe connection since you indicated that exhaust fumes were entering your car. Of course if the ticking sound has existed for some time it could be various engine noises. Haasman's explanation makes the most sense, particularly since you spilled transmission fluid recently. It helps to mention those little details when asking for a diagnosis.

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Fred Hoelzle