Can someone help me with a popping sound??

Dsiggia: Since you didn't say otherwise, I'll assume that after you removed the sub (and after you switched the system off and back on to get rid of the whine, which I'll treat as incidental and isolated for the moment) you found your noise to still be present. Taking for granted right now your conclusion about the speaker cables, you should go ahead and follow the rest of the regimen I suggested above to try and nail down the problem...

Also Sonic Frontiers support is great and the tube sets are cheap! They moved last week but give them a call or I would suggest RAM tubesets.

http://www.anthemav.com/OldSitev1/frames/tubesfr.html

I think you will be stunned by the upgrade in tubes.

Since you are using tube pre-amp, there is a very high chance that one or some of the tubes inside the pre-amp have problem.
Change all the tubes and see what happen.

I've disconnected the subwoofer cable & P/C. Now I've got an high pitch frequency coming out of both speakers. I've never had a problem like this. I turned the preamp off/on & the pitch is gone. I just purchased a new "slightly used" Sonic Frontiers Line 3 SE & added it to the system. I also added Analysis Plus Silver Oval Cables & used them for about 30 hours so far. I don't think the problem lies with the speaker cables because I've swapped them out & the problem remained. I'm leaning toward the tube preamp, but since I'm new with tubes I really don't have a clue. I would appreciate more suggestions, please.

This does not seem like an induced hum from your description of the sound. If you want to confirm that it's not your sub, disconnect it. If the problem remains, see below - if it's gone, come back and tell us.

If this doesn't seem to have anything to do with the sub after doing the above test, I would suggest swapping your speaker cable left side for right side at the amplifier, to see if the problem then travels to the other speaker. If not, then suspect the speaker. If it does, then do the same channel swap with the interconnects going into the amp. If the noise does not migrate again when you do this, suspect the amp. If it does, reverse the leads going from your source into the preamp. If the problem stays in the same channel after that, suspect your preamp (this is a likely culprit, since yours uses many tubes - this could easily be a tube noise). If it switches, suspect the source. In general, rice-crispies sounds are usually either from small signal tubes going bad, or bad connections at jacks, although other possibilities certainly exist. But try to rule these out first. If this test leads you to suspect a bad tube somewhere in the pre, just do progressive tube swaps channel for channel, swapping only one R/L pair of tubes at a time to limit your variables until you've identified the bad one if it exists. If any of this is helpful (or if it's not), please come back and tell us what you've done and found. Best of luck!...