Root causes:
The vacuum hoses leading from the intake manual have rotted out ($6 replacement) and bleed/ leak vacuum, thus triggering the vacuum pump system to make up lost brake boost vacuum.

The brake booster canister is leaking and no longer holds vacuum. Replacing it is a doable diy project ...

Had same problem with '03 2.5 AWD. Find black rubber bushing/sleeve with 4" skirt at base of steering column under carpet at feet of driver. Remove the spring clip that holds the black rubber skirt to the opening in the floor, slide the bushing down and lubricate the steering shaft with silicone ...

I had a squeek and minor shudder in the steering column of my 2003 XC90 T5 that grew worse month after month. Finally traced it to the large rubber bushing/sleeve that surrounds the metal steering column at its base at the floor of the driver's side just as it goes through the floor board. This ...