Could be a couple things at 20-30mph...
1) wheel spin can cause false knock and around 25mph is where my GN breaks loose (full boost).
2) depending on the chip programming, it could be momentarily lean. I think some chips are programmed to lean out at TPS of 1.8-2.2V to aid turbo spool.
3) could be the engine mounts, downpipe, hard downshift to first gear, loose torque converter bolts, TV cable hitting the knock sensor, or other mechanical banging going on...
Just a few thoughts. HTH.