Ok this is weird. One of the steps support suggest to test is to unplug the speakers and run the amp at full volume to see if there is a fault with the amplifier unit. The weird thing is there is music coming from the amp! With no speakers attached! Like the needle on a record player makes sound. It’s very quiet buts it’s there. 
 

is this normal? I’ve never heard an amp make sound with no speakers attached. 
 

the temperature still reached 42 deg even with no speakers attached. Idle temp is about 28 deg. 

Here are some pics of the temperature 

According to support the diagnostic I sent support  says that the CPU is running at 66° degrees. This seems awfully hot, considering it's just processing a music file,

It’s also processing the Internet connection. If Sonos support isn’t worried, I wouldn’t be either. Electronics under load run warm. 

The support chat person says it's out of spec (0-40 deg normal)  but it's not that bad. 
 

he was not helpful at all. I've owned enough hi-fi equipment in my life to know that this is not normal  - especially for a class D amp

this is also not the first Sonos amp I've owned and the other one did not get anywhere near this hot. 

I’m going to do a factory reset and if that doesn’t fix, it’s being returned. 

Neither have I. Hum yes, music no. Weird if true because I can think of nothing in the amp that can make the sound waves needed to sound like music.

If your amp is in the return window, easy enough to replace it and then see if the obtained temperatures are the norm for the product.

I only have Connects and Connect Amps, and those temperatures are common only with my Connects. 

it was a very faint sound only heard at max volume with no speakers attached. Like the sound that comes from the needle on a LP 

I have returned it now.