There's a small suction fan at the top/back with the power supply, it doesn't run when I have the case open, I guess because the system is cool enough.
On closer inspection I found that the noisemaker was actually the video card fan. It was blowing hot air out of it's exhaust slots that was too hot for my finger when I pressed it against the exhaust. This exhaust was right against the case cover.
I'm going to cut a vent hole, good idea.
I was wondering though, is there some suction cooling process that makes the air rush faster when the case is closed thereby requiring the cover to stay on?