You probably have a ground loop issue. Are the det cans interfacing with your VEMS ecu or the car battery at all (other than through the laptop)? Is your laptop plugged into the wall? Are your "mics" electrically isolated from the block? If they are grounded to the block that will create a ground loop as well. You can check that with a multimeter, just disconnect the mic from anything else that could be grounded back to the block (i.e. your pre-amp circuit).
You may want to take a look at the frequency of the noise, compare it to other signals from VEMS. I would check it against your coil firing frequency as well as anything that may show up on your TPS signal (I don't know if you can look at the raw values or not, but that would be best, rather than looking at TPS as a %).
Grounds are a bitch, and must be done right. I had a custom crank position sensor made and the guy grounded it to both the wiring harness and the housing of the sensor. So I had a massive ground loop and I would get a trigger error only when my coils fired.
Jim
