There are threads out there on MBOX noise problems and caps.
Putting 2,000uF,10V across the USB supply seems to near enough cure it, is
that permissable, wrt USB spcifications? nonly a measly 10uF or so there as
far as I can see.
There are 4 LDO regs MIC5205B, topmarked LBAA, 3 hold their outputs but one
varies 30mV timed with the hash variation, Putting the cap there makes a
vast improvement but not quite up to the main supply smoothing. Will try
tracing all caps to that reg and replace them.