Until today, I have almost never heard the fans on my iMac Pro (10-core), other than a faint background whirring. Now I am hearing them very distinctly when running Handbrake. I've run Handbrake many times in near silence. The fans are running between 1450 and 1650 rpm, temperature between 93 and 95 C., and processor between 3.6 and 3.8 GHz. I can hear the fans at 1450, but not loud, quite apparent at 1550, and too loud at 1650. The only change to the computer between yesterday and today is that I installed 10.13.4 this morning. Could that be the reason?