I know exactly what you are hearing. There could be a few things happening. If you richened the lsn and it helped but went too far and it is still doing it, there are a couple things to try. The first thing I would do would be to drop down the heat range of the plug. If you are running hot or ultra hot, try a medium plug. You could also add a shim to the combustion chamber. That sound you are hearing is a result of preignition which means the charge in the chamber reaches flashpoint before it is supposed. In regular engines, the ignition system determines when the system fires where as these engines rely solely on heat and compression. If the ratio is too high and/or the plug is too hot, it causes detonation. You could also lower nitro content but not necessary. What I would do in order is:
Lower idle and richen both needles slightly. If problem persists, lower plug temp. If problem persists, add shim, if problem persists, check front bearing for leaks but 99% of the time, you can solve the issue by lowering idle and richening one or both needles. The plug and shim change should only happen if you can't solve it with the needle adjustments. These are the cheapest and easiest tests to perform in order. Just a tip, if you have richened the lsn to the point where it loads up in 3-4 seconds after it returns to a normal idle, the hsn is either too lean and/or the idle is too high. If you go up and down the board on the needles with no luck, try the plug first and then the shim. Let us know how you make out.