MOTM

I don't own any MOTM modules, so no, it's not from MOTM! But, I have a 
few eurorack modules that I'm getting quiet ground hums from. I can 
only hear a hum from my Model 36 Leveler when I crank my distortion 
box to max (Electro-Harmonix Muff). But I can hear the A-106-1 Xtreme 
Filter hum a lot more easily, without the distortion box. However, 
there is one strange behavior that I must point out. When an audio 
signal (coming from an output with a hum) is loud enough, the hum 
disappears.

What are possible causes? I don't hear hum from my other modules even 
when I crank my distortion box to the max.

Low-quality components in the module?
Low-quality shielding in the cable?
Placement of the module in accordance with the power supply?
Does anyone hear hum from MOTM modules under extreme settings?
Are some modules just prone to hum and others are not?