I have the same problem on a C 430. I’ll be playing either quiet or loud and then a hear A rumbling noise that kind of trails off for a bit and then comes back. Did you ever find out what the root cause was?
I have a Yamaha CLP-411 Clavinova that is about five years old. It has
> developed an occasional rumbling sound that comes from both speakers.
> It only lasts a few seconds at a time, is unbelievable loud, very low in
> pitch, and sounds just like wind noise on a microphone. Seems to happen at
> random times although more common when someone is playing it. Volume
> control position has no effect on the noise volume but does sometimes seem
> to affect the timbre of the noise. It goes away when you plug headphones in
> (you don't hear it in the headphones).
> Without the benefit of a service manual/schematic, I have tried to trace the
> analog audio signal with an oscilloscope to at least determine from which
> stage the noise is coming. I'm suspecting it's originating in the power amp
> section - the amps are IC "bricks", one for each channel but since it's in
> both speakers equally I'm thinking it's not them. Due to the sheer volume
> of the noise it actually loads the power supply to a point where signals at
> all stages are affected. I did change the filter cap in the dc supply. No
> improvement.
> I did read some newsgroup accounts of noise in the Clavinovas being due to
> dirty contacts in the volume control... I'm picking up some contact cleaner
> today and will try that but I'm not too optimistic about it.
> Any experience with this problem or guidance much appreciated.
> Mike