I have black tape helping keep out dirt between my coil cover and search coil on my Gold Bug Pro. I kept losing my threshold sound while my 1/4" jack was plugged in so I used a different set of headphones in the 1/8" jack and the same thing would happen. I thought it was because my cell phone was too close to the detector. I noticed that at a higher threshold setting the sound would always stay constant. It was almost like the headphone jack had a short in it. I was prepared to tear down my detector. For some reason I thought about that tape. I took it off and was horrified to see all kinds of dirt including magnetic sands stuck to it. The coil cover had bits of black sand in it also. I'm going to use clear silicone chaulking between the coil and coil cover to make sure it's completely sealed.