Harbor Freight sells an inexpensive Mechanics Stethoscope you can use to listen and find out exactly where the sound is coming from.
Check the sound in the Vacuum Pump area.
If yo do not want to buy a Stethoscope you can use a piece of Broom Stick or something else that conducts sound placed on the part you think is making the noise; put your Ear on the other end and listen.

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