Doepfer

I have a pair of 132-3 VCAs and love them except one thing which is
making me sad. When I control them with a 143-2 ADSR they make a low
"thump" sound. This sound happens regardless of input signal volume or
frequency. the sound actually makes nice kicks ;) but makes a mess or
my delayed arpeggio patches. 

The VCAs are in exp mode. the only way I can make it go away is to use
a slower attack on the ADSR. the only other VCA I have tried is a Asol
SY02 which does not "thump". the SY02 is not DC coupled AFAIk.

I have the ADSR and the VCA's in separate G6 racks.

any suggestions? could this be DC offset due to ground issues? I do
have a furman power conditioner on the AC.