Not sure what “infuriating high pitched noise” you’re talking about. For the years I owned a pari of PLAYBARs, I never had this issue.

Have you tried to call Sonos Support directly to discuss it?

This could be anything from potential wifi interference to a possible issue with the electronics, and probably several intermediate things as well. 

Does this noise track the Volume control?

Not really

If the noise level tracks the Volume control, it is possible that at least part of noise is present in the source signal. Otherwise, PLAYBAR must own all of it.

In some cases PLAYBAR’s power supply emits a mechanical noise at a frequency that some people can hear. This noise tends to be loudest near PLAYBAR’s left rear.

In any case this is best discussed with SONOS support.

This seems to be the issue as it’s present when just idle with no music being played.

Then, again, calling Sonos Support directly to discuss it would still be the best option for you. 

That’s what I’m doing.

Hi, did you solve this issue?

Hello Rob , currently I have the same issue of a high pitch noise coming from my Sonos playbar , from research it seems to be a transformer issue which I’m trying my hardest to get fixed. 
 

I have found that these Switching Power Supply Inverter Converter Ferrite Core Custom Transformers can be found on aliexpress for about £50 for a pack of 5. 

sonos have seemed to be reluctant to fix these issues of high Pitch noise from the transformers and as it seems to be that there was a bad batch of switching power transformers that some how worker there way into supply chain during the manufacturing of these playbars. personally I find it quite annoying that they haven’t offered a replacement power supply board for a fixed price etc for users to replace them or something along those lines. 
 

I have been in contact with countless repair shops and companies to fix this issue I have currently. 
 

I am probably going to try and fix it myself but I will be calling Sonos soon to trying and see if there is anything they will do.