iMac fans run loud after hard drive replacement

I have replaced a faulty hard drive with the same make and modle, but a little bit larger capacity hard drive. While I had my iMac open, I had used canned air to "clean" the fans because they were covered in dust. After I brought my computer up, the fans began running so loud that you can hear from a different room. Everything seems to check out okay, and working properly. Any ideas what could it be?