Understanding extra thermocouple voltage between computer and machinery
It sounds like you're describing a ground loop, also an obnoxious problem in audio applications.  

Even though your computer and roaster are presumably grounded, "ground" is not perfect.  There are all sorts of high impedance paths between various ground points, and all sorts of effects, from mechanical triboelectric to electrochemical to stay fields from other conductors will cause small, subtle variations in the voltage.  In short, it's total chaos, so you have to deal with it.

In a coffee roaster I assume you have motors which are relatively noisy, and there might be an actual (very small) electrical current flowing out a vent, as all the rubbing and heat is bound to create charged particles.