Rumble question- Thorens/black widow

Hello, I've been a reader  for a while but this is my first post. Thanks in advance for any help/advice!

my turntable makes a very soft rumble sound mostly audible between the tracks or in very soft passages of music. I know the noise floor will never be like a cd but I would like to address this as I don't hear it in my previous  turntable which is a cheapie. 

I have a thorens td125mkii w an infinity black widow tonearm. I suspect the sound is actually due to the tonearm. That's because it seems to be microphonic, because if I gently rub my finger on any part of it, you can hear the gentle rubbing sound get picked up. This is the first black widow- it doesn't have damping trough.  

-do you think the rumble is from lack of damping or maybe not?
-can I diy damp the arm easily to test it?
-or what else might be the problem? It has original wiring but theres no hum or feedback at all. 

Thanks and sorry if this post was long!
-David