For the most part, it is normal. In louder environments, it's not noticeable. At a party or event, it won't be noticed. When high-gain electronics are connected to high-sensitivity loudspeakers, sometimes the lowest level of the amplifier's output, it's "idle level" is audible because the speaker is very sensitive. The reason it's audible from the horn and not from the woofer is because the horn is much more sensitive than the woofer.  The same amount of millivolts are going to both drivers but the compression driver and horn are much more efficient at converting those millivolts into sound pressure.

There is a maximum dynamic range between the lowest level an electronic circuit produces and the highest level it can produce. For pro-audio gear this is usually in the range of 110-115dB. The output of one of those horns is ~115dB with 1W @1 meter. The amplifier channel can produce 1500 watts. Essentially the horn can produce 62dB from only 0.01 millivolt. We make compromises in order to get loud, and that compromise could be the inability to get perfectly quiet.

It may be possible to make it a little less noticeable in quiet spaces, and this would also improve sound quality anywhere you would take them... (This is what everyone should be doing anyway, but it's particularly relevant here...) Try turning down the input level on the cabinet. First, turn it all the way down and see if the noise is reduced. If it is, then we're making progress. Next, put some music through your mixer and bring the mixer's output level up until it's at 98% of maximum. Yes, really high. Now, bring the level on your speakers up just enough to reach the maximum sound level you'll want in the room. With that done, you'll no longer be amplifying background noise for no reason. You won't be amplifying background noise from any source, amp, mixer or whatever. Your mixer won't struggle to produce enough output to play the speakers as loud as you want. Mixers have way more output than you need and they tend to sound better when they are delivering more output rather than less.