Has anyone any experience of these?

I have one fitted to my SL1210 and the power supply has always been inside a unit. Now I have arranged stuff and it is now on full show and I get a slight buzz/hum/from the alloy box. Nothing major but I have never noticed it before. There is no noise through turntable or the amp speakers etc. Everything can be disconnected(Apart from power) and it hums. It performs well and serves its intended use but just curious.