Krell FPB300 - mechanical transformer noise.

I have an FPB300 that does not have the pinging issue discussed on another thread - my problem is a noisy transformer. I have measured approximately .75 volts of DC on my incoming line (dedicated 30A) and plan to try a DC trap such as PS Audio's UPC200HB.
The unit makes the most noise and is most noticable when the music stops or when the music transitions from loud to very soft - unit almost growls for several seconds and then fades away. I am assuming that this is related to the class A biasing scheme. Krell had no suggestions even after I asked if DC could be saturating the transformer - they acted like they never heard of toroids and DC issues. I am feeding the amp directly with a Wadia 850 (balanced) and it is driving Apogee Duetta Signatures (Audioquest Midnight Cables) Anyone experience this or have ideas? I am very happy with the combo except for the noise. Thanks in advance for any help.