faulty blower?

car rover 45 y reg . When I turn blower from 2 to 3 there is a loud whistling sound , doesn't happen all the time , then sometimes when I go to 4 it sounds like the blower is going to stop(whistle still there) then it seems to speed up again. Sometimes if I switch back and fore between 2 and 3 the whistle will stop and blower seems to run o.k.for most of the time with the odd whistle now and again.
Do I need to take car to garage or is it something that may clear itself.
Any idea what it could be and possible cost to repair