I had this situation about 6 months ago, but seemed to find a quick solution that time. Today, however... I was recording a short section of a soft synth beat using Camel Audio's Alchemy. Everything seemed fine while recording, but when I hit Stop, the Alchemy sound continued, almost like an arpeggiator. The MIDI window in the transport showed a fast-moving series of notes constantly playing and I tried resetting there, then going into preferences and resetting MIDI drivers. No luck. I then disabled the red record button on that track and it went silent. However, hitting the record enable button on any track would cause the same issue, but on the accompanying track. This happens with EXS24, Omnisphere, Alchemy, any software instruments. I also tried some older files, and they do it now as well, as do new projects created from a variety of my templates, even the empty default. I am at a loss. The last time this happened, someone recommended an app called Onyx, and after using that and restarting twice, everything went back to normal. That was last year. This time, I've tried Onyx several times, repaired permissions, restarted dozens of times... I'm not sure whether to reload Logic from the ground up and lose a day that way, or continue shooting in the dark, which might get the same results... Any thoughts would be appreciated.

And my own response the following day... After reading online forums for hours and only repeating the same Onyx and restart steps and getting nowhere, I slept. This morning, I pulled my system apart, blew air into the tower and kicked up some dust, then put it all back together. When I opened the project I had the initial problem with, the "quirky arp" sounds were gone, but so was the Klopfgeist. My click track had disappeared. I checked the environment, dragged it back on to a new track, and was able to record with no problem. I have no idea why, but as I'm now about 18 hours behind, I'm not going to spend much time questioning the magic...

Thats very strange sammydave. So has the problem gone away now? It could maybe have been quite a few things. Most probably a corrupt Logic preference file, but it seems to maybe have sorted itself out, after all your troubleshooting.

Yeah, I think I'll look at the Klopfgeist first if it happens again...