It isn't broken. It only makes a noise. So the dealer is not liable. But he is directly liable (not the warranty insurer) for any fault present or developing on the car on the date of sale for 6 months from the date of sale. So write him a letter telling him you are worried that the noise indicates a fault with the car from the date of sale and send it by post office special delivery, then keep a copy and staple the certificate of posting to it so it becomes a matter of record. That is your guarantee for 6 months.