Joe I had to laugh at your "Honk" sound as the Excals are referred to as sounding like a strangled duck.
When you lower the coil and the threshold increases in tone or pitch it means it is positive like you said. However if it is in all metal like when you GB it, and it "Honks" it may mean you have a metal target under the coil. Also you could have some black sand inside the coil cover that sets it off when added to the ground minerals. It should remain smooth when you switch to disc.
Hope this helps,
Sandman