So....I "think" I may have found the problem. I bought two Lectrics, a step-through and a regular one. The step-through was the one with the problem and was a gift to my GF. I tried two replacement wheels that Lectric sent but the problem was not fixed. The GF was not happy riding around on what sounds like a $100 Walmart bike with the rattling/grinding sounds whenever the motor is under load.
Last night I spent about 3 hours in the garage putting my troubleshooting skills to work. I swapped the rear wheels between my bike and hers. No difference. That ruled out the wheel.
I then started going over every screw, bolt, connection point, etc... and confirmed that they were all tight. I went over the frame with a fine toothed comb to make sure there were no visible cracks. I rechecked the latch tightness.
Something caught my attention though while I was looking at everything that closely. The screws that attach the rear fender to the frame were as tight as possible but I noticed that the fender is resting directly against the metal of the frame. Metal fender directly touching metal frame = possible source of metallic rattling/grinding sound.
Rather than spending even more time dismantling and removing the fenders I decided to put a thin black plastic washer between the fender and its mount on the frame. After doing this I took the bike out for a spin and the noise was pretty much gone. I still haven't tested it under load going up a hill but after I had swapped the wheels I rode it around the neighborhood and it did make the noise. After adding the washer it did NOT make the noise.
Here's a picture showing the fender attached after I added the washer.
Based just on this thread alone, I'm obviously not the only person who has experienced this noise. I'm very surprised that Lectric isn't aware of this possible cause and they just keep telling people it's normal. Sorry but this isn't "normal". If this really is the source of the noise, it's a design flaw that can easily be fixed but a $1200 bike that sounds like you're dragging a shovel behind it is neither normal nor acceptable. I wonder how much money they've wasted sending people new wheels when it's possible that a .10 cent washer is the solution.
I guess the next question is - If this is really the cause, what is causing the vibration? I don't think it's the motor since I've tried three wheels on the bike including the one from my other bike which I know is fine since my bike doesn't make this noise. Is it just the nature of the step-through frame?
Anyway, it looks like the noise can either be eliminated or significantly reduced by adding a washer as my picture shows.
I won't have a chance to go for a longer ride and do a more thorough test until this weekend but based on last night's findings I'm guardedly optimistic that I've found the source of the noise.