This paper explores the feasibility of estimating the interior noise of a vehicle compartment when the attachment stiffness (i.e., the local stiffness of the mounting point at which operational force is applied) is altered. In order to change the local stiffness of the vehicle body, it was assumed that the local stiffness could be changed by attaching some additional systems to the vehicle body. The frequency response function (FRF)-based synthesis method is used herein to estimate the change in the local stiffness of the modified vehicle body. In addition, the change in the noise transfer function (NTF) can also be estimated using this method. Since the operational force transmitted from the source excitation to the body attachment is required to predict the interior noise of the vehicle body in the operational condition, a transfer path analysis (TPA) technique was performed. Various vehicles and additional systems were utilized to estimate the dynamic properties of the modified vehicle body. Results indicated that the synthesis technique is an appropriate method to estimate the change in the local stiffness and NTF of the modified vehicle body. The results also show that the interior noise of the body can be estimated accurately by using the synthesis technique and TPA procedure when the vehicle body is locally modified.