PSU is the box you plug into the AC outlet and it drops the 240/220/110 volts to, say, 6. You might know it as a transformer.
There will be no earth loop when using PSUs. Other than laptop ones, they are double insulated and therefore have no earth bond. The plug into the device is your clue. Two connections only means no earth. Being plastic and low voltage, the Boss doesn't need earthing. I know this from being registered to safety test mains appliances—PAT testing here in the UK. The noise is coming from inside your 300. Talk to the seller. It might be something all 300s do or you might have a faulty PSU. Don't use it until you've checked, as faulty mains appliances can be unsafe to use.