The converter/charger will supply, approx. 1.5 v more power than the batteries alone so some increase will occur. The lower 2 speeds are the , constant run settings and so designed to be quieter , while high is clear the whole unit in a hurry setting and the noise is not a factor because they don't expect you to have to leave it on high for long.
If you really consider the noise a problem , first check your charger out-put, should be 13.2 to 13.5 at the batteries, after a few hours plugged in .
I see KIX, types faster than I do. Most people do .
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.