Reason:

(1) Motor characteristics.

(2) Blower is faulty.

Solution:

(1) The specifications of industrial motors have changed since 2015. Noise may occur with earlier blowers.
If the noise occurs in systems from 2015 onwards, measure the frequency of the noise.
If the noise appears around 2000 Hz or 4000 Hz, it is due to motor characteristics.

(2) Repair or replacement of the blower is required.