Cable ch 18 is centered at 147MHz. If your cable company is using ch 18 for an analog signal you should see a video carrier at 145.25Mhz and hear a wideband FM audio carrier at 149.75. However, many cable companies have been converting their analog video channels to digital in order to offer more services using MPEG 2 compression. In that case ch 18 may be a QAM carrier, which uses a mix of phase and amplitude to send a datastream to a receiver that can demodulate and decode the signal into digital data (and ultimately a picture). These signals resemble wideband white noise due to their nature (they are sometimes referred to as a "haystack" because of how they look on a spectrum analyzer). QAM signals shouldn't necessarily produce a discernible signal on an analog receiver, just an increase in white noise.
If you are able to see or hear an FM carrier, it would indicate that there's a leak in the cable nearby. It might be in your house, it might be on the street, or even in your neighbors' house. If you're able to pick it up it's likely close by, because even with an ideal radiator it's only a few dB of signal output, and most leaks are far from ideal (although I've had a few that traveled for blocks when the lashing wire on overhead lines touched the center conductor on a cracked hardline).
If signal can get out, noise can get in, so it could be possible that your modem might be suffering from ingress. I doubt they use ch 18 for the cablemodem downstream carrier, basically because we know there's ham radio and other land mobile stations in that part of the spectrum (along with some other reasons not relevant). Usually you can check your modem settings by opening a web browser and going to
http://192.168.100.1/ and checking for a "signal" or "status" page that should tell you what frequency is in use for both upstream and downstream, along with a signal quality measure called MER (should be better than 30dB for error free communication). But ingress is usually very broadband in nature and because of the proximity of powerlines and transformers it doesn't take much to trash the cable signal. So if you are seeing problems with your modem it woudn't hurt to call and see if someone can come out and check to make sure things are OK. Don't try to explain leaks or ingress to the folks at the call center, they most likely won't understand, but feel free to discuss with the tech who comes out to investigate. They should have leakage detection equipment on their truck that can help narrow down any problems.
Good luck.
