Dave Smith Instruments SYNTHESIZERS

Hello list,

I'm having the following issue with both my DEVO and PER. 

When any of the analogue oscillators sounds at Ab4 (somewhere around
830Hz), and is applied a moderate vibrato, a crackling noise is heard.
It is only at that pitch (in a region of about +-15 of Fine-tuning),
but it is the same issue with any of the 10 analogue oscillators. 

Suppose I choose the default patch on DEVO, turn OSC2 down, set the
pitch of OSC1 to Ab4, set the amount of LFO1 to a moderate 3, and turn
up the VCA. Since the crackling is most obvious when the oscillator
outputs a triangle wave, I set OSC1 to TRI. When I turn the
LFO-frequency up to 150, the crackling becomes extreme (but disappears
when changing the OSC frequency to, for instance, G4 or A4), but it is
still audible at an LFO-frequency of, say, 75.

Does anyone here have the same issue? 
Any ideas?

Thanks!

Tomas.