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Occasional hissing & crackling noise when modem is on
Since the problem existed before you installed the Excelsus splitter it's probably not the source of the noise problem, and since you swapped modems and the problem persists it's probably not a modem defect.
Before you installed the Excelsus splitter you were probably using a simple DSL filter on each of your phones, answering machine etc. Those filters block the higher DSL frequencies from reaching the phone but don't block the lower voice frequencies and 48 volts DC on the line from reaching the DSL modem. While the DSL modem is not affected by that, having the DC voltage reach the modem may allow the modem to draw a small amount of current from the line and the amount of current may change when the modem is on/off. Some of the dedicated DSL splitters are of a simple design and also do not block lower voice frequencies or DC from reaching the DSL modem.
The hissing and crackling noise on the line is usually caused by bad connections and partial short circuits from corrosion and/or damaged wiring insulation allowing adjacent wires to touch or be bridged to each other by corrosion. In some cases this kind of noise can come and go depending on humidity, temperature, and voltage difference between the affected conductors. If the modem can draw some current from the line because 48 volts DC is not blocked by the splitter then the small difference may trigger a change in the noise just as taking a phone off hook could.
Since the problem is intermittent it may take awhile to find since it can exist anywhere from your equipment all the way back to the CO. Phone line techs have equipment that can show how many feet away from one end of the cable a short is located, and in some cases they can clear a short caused by moisture or corrosion by application of high voltage pulses to the line, or they just swap the bad pair for a good one from the same run.