A method for monitoring a condition of a piston rod of a reciprocating compressor is provided. The piston rod having a length defined between a piston end and a crosshead end. The method includes positioning a first probe with respect to the piston rod along a x-axis and positioning a second probe with respect to the piston rod along a y-axis coplanar with and substantially perpendicular to the x-axis. A current ringing vibration amplitude greater than ten times a maximum of a one-hour average and a noise floor at a ringing frequency is detected, and a bent rod condition is diagnosed.