Hypersynth Xenophone - Squeaky/ Rattling noise?

Hey guys!

First of all, I am (Was) very happy with the synth Hypersynth Xenophone.
I bought it used a couple of months ago, and everything looked/ sounded fine.

But suddenly I began to hear this squeaky/ Rattling noise or high pitch tone. Don’t know what it is, but it’s present every time I turn the Freq knob down to Lowpass the sound.

I would be more than glad if anyone could tell me what this is. Software or hardware failure?

I have attached a sound clip for you to hear the sound. It comes after the sound, almost like an offbeat note.

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