The reason for the strange whistle of the new cooler?

Question

Iron

Hello everyone
. I purchased a new cooler Arctic Cooler F9 in order to force cold air into the case. But there was such a problem: it whistles when it works for blowing (installed both on the front and on the back of the system unit). I tried loosening the fixing screws, but that didn't work. The only thing that was noticed - if you bring your palm on the other side of the grate (not even close) - there is either no whistle or it is very weak (but it’s understandable - though a small, but still, obstacle is created to draw in air, from which the whistle weakens ).
In the blowing mode and just when you hold the connected cooler in your hand, there is no whistle. How can this be and most importantly - how can I fix it?
Thanks