Trying with a battery would be a diagnostic for checking if the noise is introduced by the wall wart. Boss style wall warts, with +VE earth can often produce problems with noise. Are you using one of those newer(smaller) switching mode warts?Those can introduce a very strong signal onto the +VE earth line which would sound as you describe.
andy Rainer Straschill wrote:
My Boss DD20 has for a long time been integrated into a setup centered around a Mackie 1202 VLZ Pro mixer (aka "Nerdville Setup"). After remodelling that setup recently ("Nerdville 2.0"), I considered a constant high-pitched shrieking sound emitting from the DD20.The sound is also present if there are no inputs connected to the DD20, or if all other connections to the mixer are removed. The sound remains if I connect the DD20 to different inputs of the mixer. Trying different cables doesn't help, either.