If from speakers is less concern than the box itself. I was worried you may be having a heating issues, so glad you confirmed the source. Something from the speaker could very well be cross-talk, or degradation in a piece of gear. The next part would be trying to isolate it.
Some hardware knobs(pots) and faders will oxidize and can cause this. I quick check on the hardware would be to cycle knobs and faders several times to wear off some of this oxidation.
Cross-talk can be another alternative, or potential damage to cabling (something like a cable you may have rolled over with an office chair wheel). Events like that can potentially split insulation on the wires internal to the outer covering. Also, something that transmits RF or even an ionizing air filter too close to equipment can cause cross-talk.
Is this something that shows up in an Audio track in X3 if armed (i.e. could you record it)? How frequently does this occur?
FWIW, my 15 yo interface (which I still use as a MIDI input), has internal degradation that actually shows up as intermittent flashing on the "Signal clipping" LED (with nothing connected to it), so try to isolate pieces of hardware from the loop (or check the environment for changes which may be inserting the noise).