Traditional linear spectral analysis techniques of the vibration signals, based on auto-power spectrum, are used as common tools of rotating components diagnoses. Unfortunately, linear spectral analysis techniques are of limited value when various spectral components interact with one another due to nonlinear or parametric process. In such a case, higher order spectral (HOS) techniques are recommended to accurately and completely characterize the vibration signals. In this paper, we use the bicoherence analysis as a tool to investigate nonlinear wave-wave interaction in vibration signals. Accelerometer data has been collected from an AH-64 helicopter drive-train research test bed simulating drive-train conditions under multiple faulted components namely faulted inner race in one hanger bearing, contaminated grease in another hanger bearing, misaligned and unbalanced drive shafts. The proposed bicoherence analysis provides more details about the spectral content of the vibration signal and how different fault frequencies nonlinearly interact with one another.