First the bad news. I did get the TSB "filter" installed by Toyota (free as my 2006 Prius is still under warranty--otherwise it runs about $150.) No change. All the noise was still there.
Now the good news. After reading here about other Toyota owners with, for example, Camrys, who had a similar problem with noise when using an XM or Sirius plug and play, I saw they had bought from Crutchfield.com the PAC SNI-1/3.5 noise filter and found it worked. so I got myself one. It arrived this morning via UPS just as I was leaving for work. So I plugged it in the audio out line (that took all of 30 seconds), and presto--ALL the noise is now gone, and I get perfect sound. What the PAC SNI-1 is, is a "ground loop isolator". Anyway, that should fix any audio noise problems other Prius owners are having.