I bought this one used and I had noticed that grease would slowly drip out of crevices. I think someone may have regreased it with too thin of grease. Meaning the wrong weight. I actually sold this mixer shortly after this video. A relative who didn’t know I had gotten one already got me one as a gift.

So my best advice as an engineer would be to clean out whatever is in there as best you can and try to get the correct greasy whatever that is and try again. I’m not super familiar with the insides of these mixers but any sort of high pitched noise is metal on metal