Q: iMac Fan running constantly

Yesterday, out of the blue, my iMac's fan started running constantly. I've had the computer for a year and a half and this hasn't happened before. I went through the steps to reset my SMC (System Management Controller) and that didn't solve it. I also ran Apple Diagnostics and there were no problems detected. If anyone has other suggestions, I'd appreciate it.

Here are the specs:

iMac (27-inch, Late 2013)

3.5 GHz Intel Core i7

16 GB 1600 MHz DDR3

Thanks!

Chris