First thing you should do is stop being so timid. Your choices are two things. Get over the noises, or embarass them together and tell them that it's disrespectful to be so loud. But I wouldn't make them feel uncomfortable doing this type of thing in your house, because you don't want them getting arrested for doing it in a car. But just make sure you yell at them to shut the hell up while they are in the act so it really sets an impression with them.