CT Sean
Contributor
I too have an FDX10 / Delta 4 reg set and it "pings" on occasion. Has done so for several years and I've always assumed it was occurring in the 1st stage - I seem to recall reading the source might be the DVT mechanism. I ignore it. Have you tried breathing of the octo to see if the sound is still there? Would be a shame to replace the 2nd stage only to find it had no affect on the sound at all.