It is a common situation that a malfunction only occurs occasionally. In such cases it is difficult for the Service to locate and solve the problem as they only have limited time to check a device and its defects.
In order to avoid having to send back the device with the exact same problem, sometimes even repeatedly, you should try to specify the symptom(s) as detailed as possible (e.g. the left rear speaker gives a creaking sound at 1:04:25 of the attached DVD disc).
This way it is easier to locate the problems and there is a much higher chance you may get back your satisfaction.