Buzzing sound on loudspeakers with PulseEffects

Hello !
I have a strange issue with PulseEffects on my laptop !
When I put PulseEffects’s window on my external monitor, I have a light but noticable whirling/buzzing in my loudspeakers, it disapears if I slide the window on my laptop’s monitor of if I launch shortwave !

It is a vintage Asus:

  Host: ordi1 Kernel: 5.9.11-3-MANJARO x86_64 bits: 64 compiler: gcc 
  v: 10.2.0 Desktop: Xfce 4.14.3 tk: Gtk 3.24.23 info: xfce4-panel, plank 
  wm: xfwm4 dm: LightDM 1.30.0 Distro: Manjaro Linux 
  Type: Laptop System: ASUSTeK product: K53SD v: 1.0 
  serial: <superuser/root required> 
  Mobo: ASUSTeK model: K53SD v: 1.0 serial: <superuser/root required> 
  UEFI: American Megatrends v: K53SD.208 date: 08/10/2012 
  ID-1: BAT0 charge: 16.4 Wh condition: 16.9/57.2 Wh (30%) volts: 12.4/11.1 
  model: ASUSTek K53--52 type: Li-ion serial: N/A status: Unknown 
  Info: Dual Core model: Intel Core i3-2350M bits: 64 type: MT MCP 
  arch: Sandy Bridge rev: 7 L2 cache: 3072 KiB 
  flags: avx lm nx pae sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx bogomips: 18364 
  Speed: 2295 MHz min/max: 800/2300 MHz Core speeds (MHz): 1: 2295 2: 2295 
  3: 2295 4: 2295 

I add a LG monitor via HDMI:

~]$ inxi -Gxxx
  Device-1: Intel 2nd Generation Core Processor Family Integrated Graphics 
  vendor: ASUSTeK driver: i915 v: kernel bus ID: 00:02.0 chip ID: 8086:0116 
  Device-2: NVIDIA GF119M [GeForce 610M] vendor: ASUSTeK driver: nouveau 
  v: kernel bus ID: 01:00.0 chip ID: 10de:1058 
  Device-3: Alcor Micro Asus Integrated Webcam type: USB driver: uvcvideo 
  bus ID: 1-1.2:3 chip ID: 058f:a014 
  Display: x11 server: X.Org 1.20.10 driver: modesetting,nouveau 
  alternate: fbdev,intel,nv,vesa resolution: 1: 1366x768~60Hz 
  2: 1680x1050~60Hz 3: 1024x768~60Hz s-dpi: 96 
  OpenGL: renderer: Mesa DRI Intel HD Graphics 3000 (SNB GT2) 
  v: 3.3 Mesa 20.2.3 compat-v: 3.0 direct render: Yes 

Soundcart is basic:

~]$ inxi -Axxx
  Device-1: Intel 6 Series/C200 Series Family High Definition Audio 
  vendor: ASUSTeK driver: snd_hda_intel v: kernel bus ID: 00:1b.0 
  chip ID: 8086:1c20 
  Sound Server: ALSA v: k5.9.11-3-MANJARO 

Any idea of why ?