Help me identify humming/buzzing noise under the hood. Pictures included.

Hello all,

I noticed when I turn the key to the ON switch (second option before cranking engine - engine is off), I hear a humming sound at around the left side of the battery area, shown here:

https://i.imgur.com/hpVRtyP.jpg

Looking around, I was able to identify that the sound goes away when I pull this thing just a little bit in the direction from the arrow shown in the picture here:

https://i.imgur.com/6SaHhEP.jpg

What could be causing the humming sound? I opened the relay fuse boxes but the sound does not seem to come from there, and none of the fuses are bad or vibrate when touched.

Any help is appreciated!

Thanks!

[1999] Lexus ES300