Strange sound from Steiner-Parker filter - no Oscillator bleed

[CasioVZ]

Hi there,

there seems to be a technical issue with me new (used) MB.
Starting from the init patch I sometimes (after normally after longer usage with power on) get a strange bubbling sound from filter 1. Everything else is off (effect, modulation). I can localise the source by reducing the output level of the Steiner Parker filter. By doing so the noise disappeared. The effect is not subtile it is quite pronounced and to me it sound like a lose connection inside the filter.
What I tried to solve the problem:
- double check any other modulation
- remove any cables other than needed (power)
- check Headphone and line separately
- running the tuning routine
- switch the unit off and on -> This solves the problem until sometimes later it comes back
Any idea?
Thanks for your feedback!

[CasioVZ]

Discovered a second problem which seems to be related.: When the sound from the filter appears the metalizer is quite unstable on both osc when using it between 12 and 3 o clock. This sounds like some electrical interference.
My unit seems to be a case for the support..