I tend to think it's a 120 volt issue. Certainly, that's not certain. But, that's what I tend to think.
Approaching the problem that way, at least focusing on the 120 volt system first, is the more conservative approach as the 120 volt system generally has a higher capacity to do harm.
It's not uncommon to get a "mild" shock from 120 volt sources.
On the other hand, I still put my tongue across the terminals on 9 volt batteries as a quick check to see what shape they are in, just as my older brother taught me when I was 8 years old.
Yes, 9 volts is a bit less than 12, but I still think the comparison makes sense as these two voltages are much closer together than 12 and 120 are.
I've had many cases when I've received mild shocks from 120 volt household wiring sources due to improper wiring and leaky devices. I ran across this quite a bit when I did maintenance work on older rental properties, particularly in the winter working on old places without grounded cable systems.
A poor or non-existent bond between the frame and the ground bus in the 120V panel definitely sounds like it could be a factor. On the other hand, it's still very important to figure out where the current is escaping from the 120 volt "hot" wiring to the frame and step in the first place, and also finding why it is not flowing through the ground to the service you are plugged in to.
Starting at the source where you are plugging in is the way to start. From there, it takes a combination of knowledge, care, perseverance and sometimes a bit of luck to trace these things.
The axiom of
electrical testing is "if it tests good, it
might be good" because some faults and shorts are intermittent.
Consulting an electrician generally has value. Getting advice on a forum can result in a wide variety of responses, not all of them necessarily good or safe.
My two cents is on sale today. It's worth exactly what you paid for it.
By the way, my brother once got locked up when he contacted 120 volt power while laying under a sound mixing board some 50 years ago. He told me that if someone hadn't pulled him out from under there, he would not have been able to do it himself as his muscles and grip were locked up due to the current flowing through him.
At least he never told me to lick 120 volt wall receptacles. I
think he likes me.