Do you mean low volume at your sound system or low volume at your P5?
If from your sound system it has nothing to do with P5. Can be cables, other open inputs, some plugins add noise to be authentic the original etc.
Any instrument, P5 as well, should be set at as high volume as possible. The lower signal from the source the higher noise from the instrument and everything.
When the P5 is not used it has, as all electronics, some self noise. If I crank up my speakers I can hear some noise (from the P5) and small amount of hum (probably from the 5 m/15 ft long cable with picking up some hum from some power cable or so). It’s at the lowest readable level (-60dB) at the meter of the mixer input meter of my UAD card.
I have put a noise gate letting no noise through, just a nice clean signal. One channel noise is ok but there are plenty analog synths with long cables giving noticeable noise even if not too loud from the speakers, therefore I have noise gates for some instruments, P5 is one of these.