Car servicing

I got my santro car serviced.though the service station gave me the estimate of around Rs.8,000/- yet they raised the bill of 16,000 Rupees.Though it was not expected from the competent staff to miscalculate the estimate to this extent yet i was generous and paid the bill without arguing.then when i drove the car i found an irritating noise inside the car which was not there before nor any such noise was pointed out and mentioned in the job card by the technician/Engineer while examining my car prior taking it for servicing.On this i asked the service station to re examine the car and rectify the problem whatsoever occured while car was with them during the servicing time so as to get the car back in noiseless condition (which was the state before servicing in which i handed over the car to the service station).To my surprise these people raised another bill of approx rupees 9,000/-. Since the noise in the car wasn't there in the pre servicing stage nor it was pointed by the technician nor i had complained about it in the job card so it has to be some fault on their part which resulted in that noise. From the last 10 days i have been getting constant threats from the service station and they have been harassing me and my family with regard to the payment of second bill of 9000/- which is not my fault at all.Now they have served me an Advocates notice.Please guide with regard to the reply of this notice and future course of action.