Yes, definitely glad to hear that it wasn't too expensive to get it working right. I occasionally have this kind of light buzzing/distortion, and I still cannot definitively say whether it is my PSR-E433, or something else in my audio chain. It seems to come and go, and is certainly not as severe as the distortion in the recordings that the OP posted. Sometimes I seem to hear it with my Roland synth, too, which would imply that it is something else, like my mixer, amp, or speakers. Other times, when I hear it on the E433, I disconnect the line-out going to the mixer, and the distortion still seems present in the sound on the onboard speakers, which would imply the E433 itself has the issue. It only happens with certain sounds, and like I said, it's really not that severe. Maybe I'll eventually take it to a Yamaha service center in my area as well.
I do wonder about the mixer. It's a Samson mixer, and was not very expensive. Anyone here have any experience with these?