My blower motor was making a noise at low fan speed spettings and while braking and was aggravated by hitting bumps. When it made noise I would turn up the fan speed and it would go away. It stopped making noise altogether on its own for at least a year (knock on wood).

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T