I've recently noticed a fog horn like noise coming from the pipes. It seems to correspond with the cycling of the water softener but I'm not certain. Any idea what the cause/solution might be?

To confirm or deny your suspicion you can operate the bypass valve and unplug the system for a few days. If the sound disappears and then reappears when you put it back in service you have found the general area of the problem. The softner mechanism has several valves inside that open and close during regeneration cycles, they could have a bad washer but they are pretty complicated to disassemble. Well disassembly isn't diffiuclt it's the reassemble that has been my issue.