This has been going on since I started the game. At random intervals of about 2-10 minutes (I never counted), Portal2 will freeze and the sound will loop*loop*loop*loop. I cntrl-alt-del into Task Manager, open the game again from there and it will continue fine from the point at which it glitched, until it happens again. The reason I think this is a GPU problem is that once (once, mind you, out of like 50 times) I got a popup telling me my graphics card had crashed... I went and installed the newest driver from the nVidia website and it didn't fix the problem - all it did was temporarily mess up Minecraft.
I've got an nVidia GeForce GT 650M 2GB gfx card and an Intel i7 2.4 ghz CPU, if that helps.
Has anyone else experienced this problem / know how to fix it?