Inspect your exhaust from engine till tail pipe at Silencer shop. There are two possibilities. 1. Exhaust is torn, ruptured or has a hole a small one that is not changing your exhaust note yet but intakes water from roads and stores it in Dholki part. Mixed with silencer soot(unburnt carbon) the exhaust vapors come out as dirty. 2. If no holes found, and silencer has never been cleaned by the age of this car, knowing its a carburetor vehicle the efficiency of fuel burning is not dynamic and as optimum as that of fuel injected vehicles it is understandable for build up to increase by passing years. It is same suit that has broken loose from walls inside of your dholki area especially if something hit the silencer from below recently or if any work done in past involving removal of exhaust and refitting can also result in inner carbon gunk to come loose in bits and then expell out of tail pipe with condensed water vapor. In this case cleaning will be done by same Silencer shop. You may consult him. All in all in my opinion it is not critical or engine related fault.