Hi, sorry for the not-so-descriptive headline, here is the long specification of the problem:

  In a car kit handsfree set, there is a 2" speaker and a standard microphone. Both are connected via some circuitry to a cell phone. The whole thingy is housed in a small plastic box, which also serves as a holder for the phone. Now, everything works perfectly fine as long as the parts stay unassembled, that is on the labpratory desktop. But when we put the housing and the lid togehter, the last 2mm before closing, and also when screwed together, there comes a remarkable echo, microphone picks up the speakers signal and sends it back to the other person. The phenomenon does not seem to have direct coupling to the volume, but rather to the frequency response. Still we do not get it how to get rid of this. We have done a frequency sweep, but have not found any particular frequencies that are in resonance
Any advice, anyone...?