Toward ubiquitous vital sign monitoring, de-noising is still challenging under real driving conditions. This work intends to investigate the performance of de-noising with an additional accelerometer in different positions for respiratory rate (RR) estimation. One accelerometer was attached to the seatbelt to record the signal of respiratory movements. For noise recording, two accelerometers were attached to the car seat in two ways, i.e., (1) on the front and back side (F-B) of the seat and (2) on the left and right side (L-R). The recorded noise information was used to suppress noise in the signal and in the frequency domain. The experiment was conducted under three driving conditions, i.e., engine on, flat road and uneven road. The median of the estimated RR is for F-B 2.15 breaths per minute (bpm), and for L-R 0.93 bpm. The medians for the three driving conditions are 0.81 bpm (engine on), 0.86 bpm (flat road), and 2.53 bpm (uneven road) respectively. In conclusion, suitable positions for the noise accelerometer are on the left and right side of the seat. Further approaches are in demand to achieve a more stable estimation for the most dynamic driving conditions such as uneven roads.