I would have said you may have tuned your system to some barely audible background noise from the works, but then I would expect it to sound too bright after, assuming the works produce LF rumbling type noise. Could it actually be producing some sort of ultrasonic noise over say 15khz? Tricky to think of something that could help, but what about trying some supertweeters and putting them on at 9.30?

Sent from my NEM-L51 using Tapatalk