Catnip said:
Well, I had a look with GPU Tweak and it seems that it does throttle the card during periods of inactivity. It's just that OpenHardware Monitor doesn't show it.

I thank you all for your suggestions, and I am marking this one as solved. :)
I should have mentioned that my GPU has three fans. That means at system idle the fans may not be turning. That makes the GPU very quiet. Each of the three fans are also when spinning slower and quieter than a GPU with one or two fans. The GPU also has a large built-in heat sink that is larger than most. That helps with cooling it.

You should note that if the GPU runs too hot and the fans are not enough it may thermal throttle. That means the GPU will reduce its clock rates to keep the temperature below a critical level. In practice if you are monitoring the GPU you might see the clock rate reach some peak and then drop back as it thermal throttles. If your GPU is not kept cool enough and has to thermal throttle much you will notice it when playing games.