"It's overheating."
No, seriously, it is. If it's overheating, it's most likely going to be your CPU or your GPU, not the back of the tower. I've had experience with overheating, in fact I had a graphics card get fried. No sounds were ever made.
All you can do is hope it's not too late, and your CPU/GPU isn't fried. Invest is a better fan and/or heatsink for your CPU.
If it is fried, get a new one. If it was the CPU, invent in a case with a thermometer, or a motherboard that shuts down if the temp gets too high. If it was the GPU, get a video card that has a fan on it, instead of just a heatsink. When my graphics card was fried, I could run
Synergy ok.
In the meantime, try running a source game in DirectX 8.0, 7.0, or 5.1. Right click the game in your Steam games list, click properties, and click set launch options. Type in -dxlevel 80, -dxlevel 70, or -dxlevel 51, and try starting that. However, some games like TF2 lag like forget with DX8, and every character model is completely white.