I purchased three SM57’s within the last two months. One from Amazon, and the other two from Sweetwater very recently. I just tried the two new 57’s and they have noticeable hum at full preamp gain. I then tried the Amazon one, and the same thing. I compared all these to a 57 that I had purchased a year or two ago. No hum, or very very slight. An absolutely noticeable difference. These were tested using the same pre, cable, position, and gain in the comparison.
There you have it folks, it is the microphones after all, in this case.