Vintage Synth Repair

Hello to all.  My ARP SOLUS produces a "thump" whenever the envelope triggers.  It's not a 
mechanical noise...it's akin to tapping the end of a cable while it's plugged into an amp/
speaker.  I tried to troubleshoot by using the external audio input just to confirm that it 
wasn't the j-wire/bus bar contacts, but I can't get anything to trigger the solus (wurli, 
guitar, korg ms20 are what I tried).  I guess I need to preamplify.  Anybody dealt with 
"thumps" before?...the bad kind, that is!  Thanks--Stephen