I think I know what this random buzz is! I had the same thing with my icebox - drove me nuts for weeks.
What you need to do is to wedge something betwen the icebox and the fuse box so the icebox sits a few cm more forward in the engine bay (ie with one hand pull the icebox forward towards the front of the car and then wedge the item in there). The noise went away for me permanently. I used some soft plastic material and wedged it in there as you can see in the pic.
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