Ive also experienced this problem.

Remove the fan and vacuum the entire blower basket. Also vacuum up all debris in the blower opening. You will probably find dead leaves & little pieces of branches (from parking under trees). Vacuum the vent duct as far as you can reach throughly.

Oil the blower bearings & spin it by hand a few times. Then reinstall.

I found probably 40 dead leaves and many small little branch pieces in my fan basket. One of the small pieces of wood was causing noise when spinning a low speeds.