I've recently purchased a Yamaha DGX640 and noticed a weird problem. When I turn on the reverb effect and DSP, the sound of the live grand piano as well as a bunch of other voices would have tailing electronic noises on certain keys. The noise sounds like "wuuunnnnggg", as a lagging reverb. All the problematic keys are D and G (mainly D3, D4). However, the nearby keys don't have this problem and have a clear reverb. The issue is quite hard to notice if you don't pay attention. But once you are aware of it (like me), you can hardly ignore it.

To identify whether this is a defect of my keyboard, today I went to a local store. Surprised, I found that this also happened on the DGX640 displayed in stock. I even tried a Yamaha Arius DP and experienced the same problem. I guess this is a universal problem of some inner circuit unit produced by Yamaha.

Has anyone ever noticed this problem?

So that's what that is ... I noticed it on SoundCloud recordings!

If you want fixes to your digital pianos in the future you should email yamaha about the problem with a step by step guide. Then they will know what to improve on the next generation.

Yep I've contacted Yamaha already. They ask me to identify if this is a defect of my specific keyboard. The result is no and tomorrow I'll probably get back to them.