"and (I) always using the same couple of pieces of music."
I'd try listening (to this music) thru headphones for the mysterious "thrumming".
A friend of mine played a first-generation master tape of organ music on my system. I heard, unfortunately, the sound of the church's forced-air heating on the tape and pointed this out to my friend (he hadn't heard it previously).
The point is that this limited sample of music may be flawed (i.e., contain the thrumming noise) and you are hearing it because your system now has the resolution to reveal it.
I'd try listening (to this music) thru headphones for the mysterious "thrumming".
A friend of mine played a first-generation master tape of organ music on my system. I heard, unfortunately, the sound of the church's forced-air heating on the tape and pointed this out to my friend (he hadn't heard it previously).
The point is that this limited sample of music may be flawed (i.e., contain the thrumming noise) and you are hearing it because your system now has the resolution to reveal it.