Since the noise can be external EMI (frequency change or shift usually addresses this, sensitivity, TX power, discrimination, reactivity, and finally silencer adjustments can help), ground feedback (ground balance is set improperly or is highly variable (use GB tracking), reduce Tx power in highly mineralized soil to 2 or even 1 if necessary, and adjust ground notch (expert setting in the Ground Balance menu) such that the entire bar is black (hit the "+" or "-" sign as necessary to blacken the entire bar shown), increase discrimination, lower reactivity, and increase silencer setting above 0 as a last resort - note that reactivity and silencer setting go hand in hand, that is, the Deus frustratingly adjust the silencer setting automatically depending on what reactivity is set at, so if you want to turn silencer off or lower it for max depth, you have to go in and adjust it every time you change reactivity - on the other hand you may want those higher silencer settings if falsing is giving you constant noise) , constant falsing (again - proper ground balance, ground notch and Tx power and lower sensitivity), or even a bad coil (try a different coil to see if the noise goes away), it is hard to try to diagnose these things over the forum just based on the what you have described so far.
First, can you describe the noise - i.e., is it a continuous buzz (EMI) or chronic chatter (i.e., not continuous noise, but constantly occuring pops and crackles) (falsing). Does it happen when you are swinging the coil or independent of swinging the coil? (with coil swings, could be ground feedback or bad coil adjust as described above).
Second can you describe your baseline settings you are using and whether your ground is highly mineralized or not (the vertical bargraph on the right hand side of the screen indicates ground mineralization). If the bargraph is halfway up or more, lower Tx power to 2 or 1. Do you have ground notch at maximum as described above, i.e., complete blackened horizontal bargraph - this will not affect depth so there is really no reason not to do it. What Disc setting are you using? Are you using full tones? What reactivity setting are you using? What silencer setting are you using? What is the lowest you have adjusted sensitivity?
Other things to look at, make sure the battery is fully charged, and when you first turn on the unit, do so with the coil at waist height and away from any large metal object until the "Yes/No" external speaker prompt appears.
Looks like you have covered the usual suspects for EMI (lowered sens, changed frequency, shifted frequency - problem is this also locks Tx power at 3 which is counterproductive in highly mineralized soil).
If you haven't done so already, I also highly recommend using one of your other two coils to see if the issue is isolated to just one of the coils.
This should be enough to get you started on narrowing down the cause.
