Does the actual speed of the fan motor change or are you just feeling a difference in the source of the air flow along with a change in the sound? For example on high fan speed setting air flowing from the floor vent will be a lot quieter than when it flows from the dash or defrost vents. The vacume solonoid thingy simply distributes vacume to the various actuators to direct air flow according to the control settings.

The motor speed is controlled separate from the vacume system. No relays. just the ecc control module and a the small power unit.

I am sure the heater vacume system can tolerate a small leak, after all it does have that 4 bulb vacume reservoir. It there is a leak there why not fix it. Those assemblies are easily obtained at the wreckers. I once encountered a bad solder joint on the small PCB on mine which caused one of the vacume actuators to go intermittent.
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David Hunter