Last night as I was setting up for a gig at a local bar Ii plugged my power cord into my amp then into a surge protector and I noticed a slight hum coming from the amp. The amp was completely off, only plugged into the power source. I unplugged it from the surge protector and directly into the outlet and same noise occurred. I played the amp earlier in the day at home and did not notice that sound. Got me a little worried. Amp performed great during the gig but I was stressing that it could die on my or something. Not good since I don't have a backup and we only use PA for vox and nothing more. Anyone ever experience this? Amp is a GK Goldline 500.