Dear Rotating Guy,
Thank you for your response.
Yes it happens during normal operating condition. Start-up of machine was carried out in March, 2018 and start-up data is not indicating any abnormality. These events started after June 20, 2018.
Last year machine was tripped multiple times due to high vibration on generator. Subsequently, generator was sent to OEM where it was inspected and it was revealed that retaining ring at drive end of generator was loose which resulted in short winding. It was repaired at OEM shop and design of retainer ring was improved. Afterwards, machine vibration data did not show any abnormality till June 20, 2018.
By looking at the polar plot and spectrum, one would think it may be a case of intermittent rub (reference attached). However, it is the periodic behavior of the event that bothers me i.e. event is repeating itself after ~ 7 hours and in my experience repetition of intermittent rubbing would not be that much precise.