No, they make those noises for a variety of reasons...though it does depend on the individual guinea pig to some point. Some make those noises when they're excited, or just like what you're doing, while others will do so when they're startled or uncomfortable. Remember that guinea pigs are short-sighted, so any change in the room could put them off momentarily, and take into account your guinea pig's other body language at the time he makes the noise. Does he tense up and put his head down, shake himself and look around, or something else? If he freezes with his head down, he most likely feels threatened by something. Most other body language is positive, and remember that guinea pigs are very social creatures so they have a tendency to try to communicate everything :).