This is a fan from this kind of food dryer. I bought the dryer a month ago because of this year's abundant crops of fruit. But it was noisy, knocking annoyingly when it reached the drying temperature.
I found out the axe of the fan could jump up and down like a millimetre or two (don't know why only when the dryer got warm after some 20 minutes), making that knocking sound. So I've added an additional bearing ball to the upper cap (which was just a hole before) and added those screw nuts because the ball reduced the cap depth significantly.
Already dried two full loads of pears, now drying ~4kg of apples and it's nicely quiet - mainly the air flow can be heard now.