Very high vibration levels were experienced on a drag line when the bucket, full of wet sand,

Question:

Very high vibration levels were experienced on a drag line when the bucket, full of wet sand, was shifted into high speed for the ascent from the lake. The frequency of this peak is much lower than the running speed of the motor. The resulting vibration signature was captured in the following image. What could cause this low frequency peak?

image text in transcribed