Very long cable run question

I have a very long cable run through my ceiling to get to my rear speakers which I use for Multichannel music and movies. I notice noise, like a crackling coming from the rears, especially when the level is turned up.

It has been suggested that since I am using a 40+ foot length heavy gauge "zip cord" (cobalt cable) that goes through insulation and who knows what, that my problem might be the speaker cable.

Is there a way of eliminating this like putting the cable in a conduit or something? What about a different cable with a different geometry?

Thanks for the input - and I have not ruled out that the problem could be elsewhere in my system. I am simply beginning a thought process here while I borrow component to try to isolate the problem.

I just ran a 30 foot long set of speaker cables and am very happy with the results. I used Canare 4S11 that I read about here and other places. It is a spiral 4 wire that is 14 gauge each but when the 2 reds and 2 whites are joined together to make 1 red and 1 white it is effectively 11 gauge. It is flexible and easy to fish through walls. I used monster gold plated spades, but you could leave them as bare wire or terminate with your choice of connectors. At 67 cents a foot I think it is a bargain. It runs along 120 V AC Romex (couldn't be helped) and there is no interference. The sound is clean and full. If you buy 100 feet it's only $67 plus shipping plus connectors. Good luck!
If the problem is that your cables are picking up radio frequencies, a shielded cable could help. It doesn't sound like that is the problem since you are getting a crackling noise. You might have blown a speaker or have a loose connection. For many speakers, you can check whether it is blown (do not do this with the tweeter) by gently pushing in the cone while your fingers are spread out. The system must be turned off while you do this. If you feel or hear a rubbing noise, the speaker might be blown.

If your speakers are not built in to the walls or ceiling, you could test whether the cable is the culprit by unhooking the existing cable, moving the speakers near the amp and hooking a short cable to them to see if the problem replicates.
OzFly, They are definitely not blown, but your suggestion of moving the speakers closer for a test is so simple and obvious, I am embarrassed that I didn't think of it myself.

Thanks for the tip on the Canare Tgrisham. I will keep it in mind once I confirm the culprit.
I have discovered that my cables are not the culprit. It is something else for sure.

Nevertheless, I'd like to hear what people are using for very long runs to the rear speakers. I am looking at about 45-50 feet.