I am trying to figure out why my central air unit makes humming/vibrating sound when it's on. The strange thing is that when I open the door to where the HVAC unit is located, the sound completely stops. When it's closed, the sounds return (I had my wife tell me whether there was noise or not as I opened and closed the door). Right on point, opened door equaled no noise and closed equaled noise returning.

What is causing this issue? At first, I thought it was the filter (correct size, I confirmed), but it's not.

Any advice would be greatly appreciated!

when I open the door to where the HVAC unit is located, the sound completely stops
What does that mean? Does it mean that the unit sits in a closet and when you open the door to the closet the sound stops?

Or does it mean when you open the door of the unit the sound stops?

Sounds like insufficient return air and opening the door allows better flow.

It could be anything from a loose door latch, to a mounting bolt. Does the sound stop when you put your hand firmly against the door? Feel any vibrations? That would be the easiest place to start.

Let us know what you get.

First assumption is you have the heater / air handler in a closet and that the closet door has a vent in it. Check to see if that vent is humming. I have seen similar issues that require a slight bend in the vent vanes to stop the humming. Opening the door eliminates the air flow through the vent.

Sounds to me that your filter needs to be replaced.