Noise and ingress are the two different animals. You can have an ingress without noise and a noise without ingress. But of course on bad / damaged cable you'll have both
Ingress is when foreign signal is getting into cable system -> over the air broadcast, CB radio, air traffic control. If affected you'll see: double picture, shadowing, horizontal or diagonal multiple lines (as opposite to hum - one or two horizontal slow moving lines). Sometimes you can hear a buzz or sound doesn't match a watched channel. Ingress can be found on any channel but it will affect single channel and not a whole band. So for ex. chnl 9 is bad but 8 and 10 are perfect, or chns 5 and 9 are affected by two over the air chns but 4, 5, 7, 8, 10, 12 are good.
Noise usually affects low and sub low bands (5 to roughly 50MHz). And is visible as a "snow" without any pattern. Noise can be caused by: lose connector(s), metal wall-plate, copper braided cable (common in older houses), rusted connection points, water damaged cables, bad shielding (also cause ingress), "Radio Shack" splitters and amplifiers, bad grounding (which also can cause a hum).
Checking channels from 2 to 13 is misleading because modems don't use those bands. Your troubleshooting scheme is perfect. It is good idea to check chnls 2 to 6 when modem is having troubles and when is OK. There is no point to check chnls 7 to 13 it is better to check 14 to 22. All is about bands:
Low band: from 54MHz to 84MHz -> channels 2 to 6
Mid band: from 121MHz to 169MHz -> channels 14 to 22
High band: from 174MHz to 211MHz -> channels 7 to 13
(some sources indicate slightly different border frequencies. The best is to use FCC as referral. I used: »www.jneuhaus.com/fccindex/cablech.html
So if there is a noise messing up then most likely it will not go higher than ~150MHz. Ingress from TV stations should not be a problem besides of harmonic frequencies. CB radio might be a bigger problem because it ranges from 26 to 27 MHz and it's very close to modems upstream frequencies 25 and 32 MHz (in some areas those freq might be different).
Above issues affect modems upstream. Downstream (around 600MHz) is usually affected by bad wiring or strong EM field from high frequency equipment and distortion. By bad wiring I mean for ex. lines made of different pieces of a cable with different values of VOP (velocity of propagation). Those mismatched pieces work similar to a prism "spliting" a light. In better piece the signal is accelerated and stretched then in worse one is squeezed. And when the signal reaches a modem, even if it has right strength, it's out of shape. Modem will see it as a "noise".
Sorry for long post in similar to English language
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