well it was obviously dubbed in afterwards and not right there when they were filming as the noise is not coming from her mouth
also through out the movie like when terry is being called into the office for taking to people on the computer cynthias character you can hear saying thingsshes sorted it etc and its not terrys fault ..along them lines and you can see her at the side of terry and her mouth is not even moving
this happens a lot in the movie and like the scene at the party they just dubbed extra speech in after filming which is common

That's right Tracy Ullman is making that noise.

errm nope the noise was not actually coming from her either plus she had already moaned at him already her mouth was shut.
the noise happens when terri is stood with her mouth wide open but it sounds like the noise was added on afterwards

It was tracey ullman whimpering at her husband.