I seem to recall a post about hearing static along with the harmony(vocal) on the PSR2100. I experienced this for a short while in the beginning, and then it disapeared till last night. I found the culprit to be a cheap "dollar store" power outlet strip that was provided. Must have had lousy sheilding, and in a digital world ..... interferance can be very annoying.
Anytime an external power supply is used (wall warts, lumps-in-middle etc)..... the potential for hum and static seems to increase depending on placement.
I remember having a similar problem with the Boss SE-50 effects unit. When ever it was racked with other gear .... the AC lump had to be positioned in just the "right spot" or the noise was unbearable.
Food for thought ........ don't skimp on the cords, power strips and such. They DO matter in certain conditions. You don't need to have monster cables to get a great sound, but you DO need good, solid Hi Q gear to reproduce digital audio with the pristine quality it is capable of. Savings are not always measured in dollars. Sometimes, it's the SENSE that counts the most.