Unform periodic print defect same in X and Y axis  

Unform periodic print defect same in X and Y axis

Hello All,
I  just got my mini and it is working fairly well, but has a strange problem I do not see mentioned anywhere.

All prints have periodic defects that line up along the print and the defect is even seen in the priming print line that is printed before the actual print starts.

Here is an image of the priming line (or whatever it is called: the line at the bottom of the build plate printed prior to printing the actual part):

Here is the printed part:

I can hear a subtle thumping at about the correct rate to make those defects.  I placed my finger on the print bed and can also feel a subtle motion happening at about the correct rate.

Troubleshooting steps so far:

I though it may be a bearing issue in the Y stage, so rotated the part 90 degrees and reprinted.  The part looks identical, so also applies to the x axis.

I thought maybe it has something to do with the extruder, so unplugged the extruder motor and printed (without actually printing) and I could still hear and feel the periodic vibrations.

Reset to factory and repeat setup and print again.  Passes all tests but the printing problem is still there.

Check grub screws (set screws) and they are tight

Belt is taught

Since the defect is every 5mm, I shifted the part by 2.5 mm in X and Y.  The defect followed the part and was not at the same plate x,y locations

Printed the part at 150% and 50% speed.  The spacing of the defect is still at the same location and every 5mm.

I did flash with 4.0.3 when the mini first arrived, so am using the latest FW for all printing.

I put the g-code file on the USB drive that came with the Mini.

I also generated g-code for my MK3 and that printed perfectly.  This problem only shows up on the mini.

Since this shows up in the "prime" line, I don't think it matters what software generates the part unless the g-code also controls that line?
Any hints at what I should try next?