Jane Loughnan, of Rangi Ruru Girls School, asks :-
Why do modern fridges "hum" more than old ones?
Geoffrey Chase, a mechanical engineer at the University of Canterbury, responded.
There are a variety of reasons a refrigerator hums. The source is typically a vibration from the machinery or an interaction between machinery and refrigerants, creating motion in the air which is sound. There are therefore several possible causes for these sounds.
The pump used to pump the (cool) refrigerant through the fridge or move air in the fridge, and the pump used to cool this liquid is one possibility. Pumps are essentially motors with rotating parts which can vibrate if not made precisely round. Hence, cheaper manufacture of these parts may be one reason.
Sound is tranmitted quite well through some materials and air and is blocked by others, called sound insulators. So another possible cause is when sound insulation is skimped on in order to save money and/or weight.
Refrigerators today are typically bigger than older ones. These require the pumps to work harder and hence their hum tends to be louder. The same effect can be caused by keeping the inside of your refrigerator colder or having more things inside it. Each requires more effort and fridges today tend to be kept colder and more filled than in past.
Pumping at higher pressures can also cause the tubes that the refrigerant fluid flows through to vibrate. This is particularly so if cheaper, more flexible, plastic tubing is used, instead of steel tubing, in order to save cost and weight. Additionally, bigger fridges may well have longer tubing, as well as more of it, given that modern fridges try to keep the inside cold as evenly as possible in contrast to older models.
Those are some possible causes, all centered on vibration combined with cheaper materials and lower quality of manufacture, as well as those changes brought about by consumer demand for different features and better performance.