Of course you can lodge a complaint either during or after a Cambridge exam. It’s even encouraged during the sound test before the Listening Paper, especially as you can’t interrupt the listening. In other cases, the best idea is to complain immediately – in writing if it’s really important and needs t reach the authorities. If you “remember” that the room was too noisy on seeing your exam result, your complaint is not going to sound too well, isn’t it?
If anything objective occurs (the sound system doesn’t work, a bird flies into the room, an invigilator dies of a heart attack), Cambridge is sure to give you another chance or some extra minutes, depending on the situation, so don’t worry.
Also, if you have any special needs that are documented, let your centre know while signing up for the exam, so that they can help you. The help ranges from wearing headphones during the listening exam to being in a separate room to being able to write your writing exam on a computer.