The heater blower noise seem quite bad. It is possible that a spherical bearing is fudged on the motor shaft and can't follow at full speed.

Many years ago I posted some pictures showing a cut view of the motor inside. This is for the new motor type starting around 1985 may be. Your motor is a little different on the outside. The casing has 3 legs and it is held in place with rubber bushings while the new type motor is inside two half moon shells with foam cushions in between. The motor inside is the same. The pictures show where to drill holes to bring lubricant in the right place. I could send you cut view of the older motor style if you need it. The right side hole drill location is a different picture but the left side is the same. If you use solvent to release fudged bearings it must be NON FLAMABLE.

At the mileage your car is, the motor bearings and brushes/rotor and commutators are far from being worn. If nothing has quit inside it could be lubricated ( I mean the two brass bearings are still held in their motor cover cavities with the springs). If the crimped washer pushing the spring that pushes the bearing in the motor cover is loose then the rotor bounces around on the stator. Because nothing hold it centered.

I am sorry that the pictures were too large to fit well with the text.

You can see pictures of newer motor cut views at bottom of page 20, sept 22 2017. You can reach that page by pressing many times "show more" at the top of this page. The post title is "Squealing interior vent fan" at bottom of page 20.

I did lubricate my 1982 and 1991 240 and it worked well. Center console must be removed and all wires identified. I used a 1 3/4 hole saw to cut into the plastic heater box. For the motor a sharp point 6 inch long bit is required to start drilling the holes and use a 1/8 inch bit to finish. Not everybody has
this kind of equipment.

The post title is: brickboard.com/RWD/volvo/1649374/220/240/260/280/squealing_interior_vent_fan.html
Prepare to replace the motor if something is broken. Do not forget to visit Art Benstein's site which is "cleanflametrap.com". It is fabulous.

Good luck Rolandp