I've run across an interesting problem with a device I'm building. It seems that when I put my PCB into its plastic enclosure, I can create some signal simply by tapping the case. What makes this predicament even more interesting is the fact that when I put a silicone protective casing atop the plastic, the effect manifests itself differently.
This is a picture of what my signal looks like when I tap on the casing.
Could this be a mechanical issue perhaps?
In addition, here is a schematic of my sensor circuit pre-amp if it might provide additional clues. The use of such high resistors for my biasing circuit is necessary because my sensor produces very small signals. Of course the downside to this is that I am less immune to noise... could this be what is causing my "tapping" issue?