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From what you are describing here, it sounds like the bearings internal of the blower motor are failing. The blower motor is accessed from under the passenger's side dash. Remove the blower motor and inspect for any sort of debris that may be touching the fan blades. If the visual inspection checks ok, you may be able to spray some WD-40 inside and lube the bearings. If this helps but does not solve the problem, replace the blower motor. I've wrapped up a small PDF file that should help with the location of removal of the blower motor. Download the PDF file here: https://www.dropbox.com/s/u8iobkeoaiwhmh5/toy-blomo.pdf?dl=0
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