Induction unit makes buzzing sound

I was fed up with the low quality electric hobs which are installed in my 1 meter wide "kitchen" (I forgot a crepe on the smaller one on the highest setting, and 25 min later it wasn't even browned, just dried a bit), so I recently bought a small induction cooking unit. It is a big improvement in terms of cooking convenience.

However, I noticed that with most cookware, it produces a strange buzzing sound. I don't mean the sound of the fan cooling the electronics. It produces a distinct fan-like sound, and I can clearly hear it when it works (it is self-regulated and doesn't blow all the time). But the buzzing occurs independently of the fan. It sounds like an old-fashioned phone, but not as loud and a bit muffled, as if the phone were under a duvet two rooms away. It definitely isn't supposed to be there (such as a warning buzzer). The device communicates its state primarily through a small number-outline display, and it also produces electronic beeping sounds to confirm the press of one of the touch sensors (really, who thought that this was a good interface for anything?! The thing is so unsuited for everyday use, they had to add annoying acoustic feedback because the normal tactile one is missing. But my accidental activation rate is still well above acceptable on all devices using it. [/rant]) and also to catch my attention if needed, such as when the overheating prevention mechanism turns the heat off. No buzzer of any kind is mentioned in the user's manual, and anyway it would be too 20th century to be included in this design.

The buzz isn't constant, it continues several seconds, then stops, then appears again, and so on. Sometimes it is completely missing. It seems to be dependent on the weight of the cookware. Once I cooked with an enamelled dutch oven, and I had no buzz at all. But with all other pieces of cookware I tried, I had the buzz. These include: a cheapish stainless steel pot, I think it has a plain thin stainless steel bottom; a good quality stainless steel pan with copper-sandwich bottom and mineral-based nonstick coating; a plain cast iron pan. I have noticed that the buzz seems to stop when I fill the cookware with enough food, and as the dutch oven weighs more than the other items, I think there might be a weight relation.

3-4 years ago I had access to another portable induction unit, and never heard such a buzz. That one was a vintage model from the mid 1970s. So if the buzz is normal for induction cooking today, why did they drop the non-buzzing technology used in this old piece?

I found an explanation on the Internet which claims that the buzz is caused by microscopic vibration of the layers of a sandwiched bottom. This cannot be the case, as the cast iron pan definitely has no sandwiched bottom, and yet it does generate the buzz.

Does anybody know what causes the buzz? Is it a malfunction of the induction unit, or is it normal to have it? And, because I find it somewhat annoying, do you have any ideas how to prevent it?

EDIT: I don't believe the explanations I have heard till now. "Poor sandwiched construction" is out of question, as it happens with a pan cast as a single piece. "Bottom vibrates against glass" sounds OK at first glance, but I put the pans on a thick fluffy cotton wool pad, and it didn't even reduce the buzz, so it couldn't be the reason. As for the fan - the sound is present before the fan switches on. Still, I am choosing an answer as the accepted one, because it tells me that it isn't a defect in my unit, and this is what I needed to know most.