No Detection Tone under 3"

[Darin]

On my SM Pro kit when I pass a coin over the coil I have normal audio tone above 3" and can pick up a US nickel around 10" ( first detection good tone at 8"). But when I pass a coin over the coil below 3" I do not get the high tone detection, I just get a thump sound. Tried different coils that work on my MPP but no difference. It just started this one day awhile back but I have not messed with it in a while, moved on to the MPP build but know would like to see if I can get it working .
Any ideas what could cause this ?

Thanks

Darin

[FatBob]

Might be too much gain. The gain was doubled between the Surf and Surf Pro.

[Darin]

This is something that just started one day. It worked when I first got it then one day it started the thumping sound under 3".
If it is the gain what would cause it to change on its own.
This is not the surf pro from White's it is the kit like cddz had. mine came assembled.