Replacing a windshield is not such a specialized job that one needs to go to the agent. It is a known fact that all car agents fleece you. While installing the windshield, you may have noticed the workshop people using silicone sealant during the process of installation. I guess that at some point less quantity of the sealant was applied or was not applied at all at that place, hence the air flow. Correcting the situation may require the windshield to be removed completely and the gap refilled. Be present while they are carrying this out, as I fear there may be some damages to the windscreen while there may not be any if the workshop people are careful and carry out the work properly.