Hi Ian, we have also experienced this issue, and unfortunately we don't have any solution to this now.
about 5 years ago
When I play a MP3 file recorded at 48khz after playing one at 44.1khz I get a very loud, short (~0.1s) burst of speaker blowing noise at the beginning and end of the file (assuming the next file is 44.1khz). I know the noise isn't an artifact embedded in the actual MP3 file as there is no noise on the analog output. It looks like the noise is coming from the DAC in the amplifier when it changes rate to 48khz. Is there anything that can be done?