I dont know what equipment your nunning in the field, or what your bandwidth is for your return. What is the bandwidth of your transmitter/recievers? And your field equipment? You say guardband so I am assuming your field equipment is 5 to 40. Trans/rec may have a higher range. Should help you in tracking it down.
What I am getting at is you should have diplex filters that prevents noise from the field getting to your headend in the guardband. Have you tried replacing diplex filters at node, or possibly node module?
I have had similar issues where the diplex filter took a crap.
Just a thought. I can only hope it is something that simple.
Or the easy way is to make your stop freq. 40. hehe Let us no your resolve bro.
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