Sounds like a “grounding” issue. Should be a ground wire located somewhere on your turntable.

Could also be defective cable or incorrect setting for the phono cartridge MM or MC.

A poorly aligned cartridge can also cause problems like distortion or a humming sound when playing records.

Thanks Aj, but this is why I tried directly connecting another unit (CD player, no grounding required with own cables) to check if this was an issue with the turntable, pre-amp or cables. There is no music coming through from the unit at all, just the buzzing. I’m wondering if a fuse would cause this (just affecting the line in section) and if so, is it something I can try to repair? It’s still under warranty but I’m happy to try if it’s a simple fix!

Make sure that the RCA plugs are fully seated in their jack.

As a test, remove the power connection to the CD player. The CD player should be fully isolated from everything except AMP.

Thanks Buzz, tried this but still no luck. It only happens when somethings plugged into the amp. Tried different cables too. When I switch back to music from app it’s totally fine.Might be time to submit a service request :( 

If you connect AMP’s left and right inputs with a single RCA cable do you have hum? Use the shortest possible cable and bundle any extra.

Hi

Going back a bit….
 

Assuming the cables aren’t picking up interference (which is why I assume @buzz suggested a short single line-in  RCA cable) then it may be a defective Amp.

How long have you had the Amp? What is the return policy from the retailer (Sonos is 45 days). Sonos does not offer service only warranty exchange.

I suggest you call Sonos tech support.

1. You have the option to run a diagnostic within 10 minutes of the occurrence and post the reference ID in your next post so tech support can reference it when you call.  Diagnostic: Go to > Settings > Support > Submit Diagnostic 

2. You can call Sonos tech support and they will most likely have you run a diagnostic while on the call. (I suggest option 1).

Note: Since this is not a network issue Tech support may not be able to diagnose the hardware (Amp). If you are in warranty there should not be an issue with an exchange. Even if you are out of warranty Sonos generally offers some type of monetary relief based upon circumstances. 

“Shorting plugs” would be the best choice for test inputs to Line-In, but regular folk don’t usually have shorting plugs available.

Another test would be to physically move AMP slightly. If there is a change in the character of the hum, then there is an external field that’s causing trouble.

If you have an RCA cable to sacrifice you can make shorting plugs by snipping the cable, stripping the conductors a bit and twisting them together.