I also have a glass-top stove at home. So far, I really like it: the heat is very even and there is good heat transfer, provided the pan makes good contact with the surface (one skillet I have has a bit of a lip, which makes it heat more slowly).
I've slapped skillets and pots full of water around on mine, and have yet to break it. I'm probably more careful than I would be on a metal range, but it seems sturdy.
But to your points:
1) There is no way cast iron could melt the glass. Iron melts lower than does glass (1200*C vs. 1500*C), so before you manage to melt your stove, your pan will be a puddle. It may, however, be possible to deform the stove top if you let an empty pan heat for some time. I don't see why this would be more of an issue with cast iron than with any other piece of cookware.
2) A heat defuser will work fine. Don't get the kind that is designed for use with gas, but anything else should work fine.
3) The burners on my stove are smaller than my skillet, so I find I need to move the skillet around to heat the edges. Also, flat-bottomed pans and pots seem to be much more effective on the flat glass top, as conduction seems more effective than radiation here. I polish the bottoms of my pots to get better heat transfer, although this is just me being anal.
Also: cast iron can't scratch glass. Glass is much harder than iron (see this wikipedia article), glass having a 6-7 on the Mohs scale, iron having a 4.
As you probably know, induction burners produce an alternating magnetic field that induces a current in the cookware. This process can produce attractive and repulsive forces as well that cause vibrations.
The problem can be caused not only by poor sandwiched construction but by an uneven bottom surface, or loose parts or lids.
I have read that some people have successfully used silicon baking pads to dampen the sound.
This page has some useful information on the subject:
http://theinductionsite.com/proandcon.shtml
Best Answer
Glad you are getting this checked out.
As Steve mentioned, it sounds like the power supply or the supporting electronics for the hob is starting to fail. To be on the safe side, I would isolate the power to it if at all possible (UK hobs are required to have a wall mounted isolation switch).
Certified electrical appliances are designed to "Fail safe” so the risk of fires etc. is small, but your hob is sending you a clear warning sign that it is not very well and should be turned off. Depending on the nature of the fault, this may eventually result in absolutely nothing at all (apart from mild irritation), a totally dead hob, a small bang, or indeed a big bang. The greatest risk you are probably taking is that your insurance company will not cover you should something adverse happen and they prove you continued using the device despite being aware of it malfunctioning.