[Elecraft] Noise going up with reducing XTAL filter BW

[hb9ari]

Hello,

Can somebody explain to me why the "audio" noise
increase when i select a lower BW XTAL filter?
For example going from 1k8 to 400Hz.

As i use often BPSK31, the "blue" background
of my waterfall (without signal present) go
into the "yellow" color; (using Digipan with
"standard" waterfall colors).
I was expecting a "constant" level or a noise
reduction, but not an increase.

Is it a Filter gain setting to modify in my K3?

Mny tnx in advance!

73 QRO de Rudolf, HB9ARI

PS At 66 yo, it's not to late to learn something!