Linear power supply with buzz - isolated transformer from case

So I got this 100VA linear power supply from aliexpress. The specific builder/brand was recommended by a youtuber and I needed a dual voltage device for my usb galvanic isolator 9v and my geshelli dayzee dac 12v.

It had a noticeable buzz that got louder when I put my ear closer to it. I was pretty disappointed and tried to use aliexpress's "free return" program. The seller disputed my reasons and prevented me from returning it. He refused to admit it was possible it buzzed. Said I must be overloading it. Said all transformers larger than 80VA have a buzz. Ignored that I have an antek 1000VA balanced isolation transformer that's completely silent. In the end I got a partial refund of $25

I took off the top panel of the case, to see if the transformer bolt nut was loose, I was planning to tighten it. The nut was already very tight. I noticed there was no noise at all when I turned it on without the lid. When I put the lid back on I noticed it came into contact with the top of the transformer. I thought the transformer must be transmitting vibration into the lid causing the buzz.

Long story short I ordered a 4x4" by 1/10" sorbothane sheet from ebay and put it in between the transformer and the lid of the case and it seems to have worked! No more noise. Hopefully this doesn't make it overheat, it was already pretty warm

 

Kind of reminds me of placing a slice of cheese on a cooking hamburger patty

 

The contact with the lid probably made what is called a shorted turn. Essentially, the bolt through the middle and contacting the case completes a new winding that generates voltage/current and can overload or saturate the core, making it run hot and noisy.

So it might not be physical isolation you've fixed, but electrical isolation.

 

I'm with @Beefy. Did it make noise with the lid off? If so, maybe it was a trafo with loose windings which can vibrate due to magnetostriction.

 

zero noise when I ran it with the lid off. I bet you guys are right. Either way it seems solved with the sorbothane sheet in there!
edit: @Beefy it seems to run cooler now too! I bet you're 100% right about shorted turn

 

The little voice in my head says get a non-ferrous bolt to reduce any potential magnevoodoo.

 

Aye