Paul – good problem and the solution you used is good information to share with our members. The approach you used to isolate the source of the noise is a good one – especially when you consider that we will all experience this type of problem at sometime in our life.
Broadband noise sources can be caused by a wide range of sources both inside and outside a home. Poorly designed switching power supplies (i.e. low voltage DC wallwart supplies), arcing AC wiring switches and contacts, HV AC power line leakage or low level plasma discharge, and/or poor NETWORK wiring can all be a potential source for this type of problem.
Most of the noise sources we’ll encounter will usually be broadband due to the rich harmonic content associated with transient and/or non-sinusoidal signal (digital) sources. The use of an old AM portable radio tuned to an unused AM channel near the low end of the AM dial can often be helpful in locating the signal source. Note: As an aside, monitoring around 500 KHZ at the low end of the AM broadcast band is a good warning of approaching electrical storms.
Your addition of an old DSL filter to your cable network phone line was an effective solution and I would like to add that clamp on ferrite RFI filters might offer a good working solution as well. (It is usually helpful to have a couple of clamp on ferrites your shack and many major electronic part suppliers stock them).
I searched the web to learn more details about the internals of a typical DSL filter and stumbled across a couple of interesting articles. W8JI documents a slightly different problem where his 160M operation would crash his DSL network. The details of his problem and a low pass filter that he reportedly added (to prevent his RF leaking into his DSL network) offers another interesting RFI prospective. You might enjoy reading W8JI’s “My ADSL Modem Filter” for future reference.
Again, good information and thanks for sharing your experience with the forum readers.