Well as I can hear the distortion in the recording, which was done through a USB connection, that rules out any external sources of vibration, such as a music stand or loose cables. In the second recording, are you saying you cannot hear the distortion in the third set of notes, around 15 seconds in, when the reverb is turned all the way up?
The settings to try is simply what I listed above... set the basic piano using the portable grand button (which turns off all special effects), then set the reverb type to Hall3, then hit a single note repeatedly, holding it down for a second or two each time, while increasing the reverb amount using the live control knob. I didn't try notes throughout the keyboard, but clearly, I can hear it at least in the middle range of the keyboard.
I'd be more than happy to try it on another PSR-E433, except that I know of no other one within miles of me! I'm sure someone has one, but I have no idea of who. But, at the music store, I did NOT hear the distortion on the PSR-E463.
I'm wondering if it is just the reverb circuit is just going bad. And on that note, I wonder if there is just simply a chip in the keyboard that manages the reverb, so that simply replacing that chip would solve the problem. I did notice that I previously had the main voice level set at 122 (I lowered it to 100 for the samples I recorded above). Maybe I just overloaded the reverb chip and burned it out.
I played around with the E463 again today, and while I like its features, I have to agree that the key feel is not up to the standard of the E433. In fact, there was an E263 sitting right above it which actually had a slightly better key feel than the E463. Almost tempting to consider a Frankenstein maneuver by replacing the key-bed of the E463 with the one from my E433. Wonder if that would work! Of course, even if it did, that would shoot down the warranty!