I pull my face , and grunt when the going gets tough- and I'm not using weights that are that heavy- I'll probably shriek with vigour when I can squat double my bodyweight .
There was once a bodybuilding book that I had (either by Ellington Darden, or called 'Brawn')where it was advocated that one keeps a straight face at all times, and no shouting or noises. That's bloody stupid and dangerous advice in my opinion.