What they are hearing is the normal sounds of the defrost, probably. Customers tend to pay attention to every little sound, particularly the elderly who stay in the kitchen al the time
On the other hand, there is a chance it is just the contraction of the eveaporator coil if the sound is within 3 minutes of startup. Perfertcly normal.
See my forum topic on the subject:Refrigerator Noises and What They Mean
Of course, I did not watch you replace the compressor, so I am ASSuming you replaced the filter dryer as well. This is S.O.P. with Frigidaire, as well as a sweep charge to remove any impurities from the system. There is a video on this in the frigidaire section somewhere. If you did not follow procedure, chances are that air is in the line, which will quickly destroy the compressor and foul the entire sealed system.