Your fan sounds like it's on it's last leg. It could be the voltage not being supplied to the motor due to old age or resistance. Or just that the motor isn't generating as much power to spin the fan.
Either way, it doesn't sound like it's going to last much longer. If it were me, i'd skip the pain in the neck and drop $30 on a hyper 212 evo and never look at the stock POS cooler again.