This thesis presents an advanced methodology for detecting gear tooth surface defects in the reduction system on an electric drive unit (EDU), with a specific focus on the transfer box (T-Box) integrated into the eCD-140 model, developed by FPT Industrial. The study was conducted on an end-of-line test bench, with the primary objective of establishing an accurate noise threshold to identify defective units. The initial defect detection approach led to an excessive number of false discards, necessitating a reassessment of the selection criteria. To address this issue, a refined methodology was developed through a systematic analysis of previously discarded serial numbers, thereby highlighting a final comparison respect to the initial technique.