ARP Omni II - thumps on bass output

Hi ARP enthusiasts,
I am working on an ARP Omni II - a range of issues - primary one I am hoping to get some help from somebody with a working unit. Could somebody verify the following..
Using the dedicated bass output, 8 or 16 bass selected, bass volume down, is the unit silent when pressing any of the bass keys? (1-20)
My unit has a noticeable DC thump on the bass output for a few milliseconds in this configuration which also flows through to the output mixer if not using the dedicated bass output jack. As the bass volume is raised this thump still exists until eventually is way below the bass sounds. It just seems strange for this thump to exist at all.
Any clues appreciated!
Cheers