Even though opening the lid makes the GPU work harder, it typically is a win in terms of cooling the laptop down. Also, try plugging in the power adapter on the right side USB ports, that allegedly makes a difference in heat
Seems to play nicer with my other (Dell) monitor but when I was using Garage Band (only on the external monitor!) it got so loud I had to quit GB and let it cool down... and I still never went over about 20% CPU. The fans are pretty normal when I only use the built-in monitor, but the case still gets way too hot to rest on my legs.
I'll try the right side power, thanks for the tip!
Thanks a lot for the tip! Funny, that's how I had it connected with the previous monitor and I didn't have trouble with the clamshell mode.
FWIW right-side power didn't help, nor did the turbo switcher AFAICT (I'm not doing anything CPU-intensive) -- but as soon as I put the monitor in the left and the power in the right the fans dropped way down. Still running, which is annoying, but not especially loud.
I find it maddening that a $3600 computer has a problem like this, and Apple doesn't even tell you about it.