Ok, I'll admit to faking my moans sometimes. But I do it to get the girl to work hard, thinking I am about to cum; and then I back off, so they work even harder. Many a HJ gets turned into a bbbj this way, and a hj/bbbj into a cowgirl, not to mention the tits out pants down in a normally staid rub and minimal tug.
Moan on mongers....