Moerch DP-8 tonearm?

There's always the risk there is a item supplied from the so called 'Friday Effect ' where assembly is the work being carried out. 

Worker become blind to certain parts which become discovered as not having been added to an assembly.

With Tonearms and as seen the same with Cart's, the more who are used to assemble, the more variations of an End Sound will be produced.

Items that look the same, but produce Sound with a discernible difference.     

Where did you add 7g of weight? I hope that if you added it at the headshell end you then re-set the counterweight for proper VTF. If effective mass of the arm is too low for the compliance of the cartridge then in theory the resonant frequency can be high enough to occur within the range of bass frequencies on an LP. That could set off the problem you describe. As I recall, Moerch make a selection of arm wands that are interchangeable and differ in effective mass. If you’ve got the low effective mass arm wand (meant for use with high compliance cartridges) and you are using it with LOMC cartridges (which tend to be low to very low in compliance) perhaps that is your problem, and that’s why it was cured by adding mass.

+1 ^

I've added the 7g rubber weight in the middle of the arm tube, and the resonace completely disappeared. And of couse I adjusted the counterweght again for the proper VTF!!

My arm tube is the blue one, that is, the heviest that Moerch produces (14g).

My interest here is that I actually have one for a future project.