Hi - I don't know if you've solved the problem but for me I have to restart the server computer then restart the server / re-load my project etc. It happens randomly after many days of use and just restarting the server isn't good enough. Works fine at first, go away for a while and come back and popping sounds each time I start playback.
Play the sounds by clicking directly on Synchron player keyboard and no popping sounds. While Sibelius is still connected, I reloaded the instrument into Sychron player - I loaded VelXf version of the preset then reloaded basic preset and it still pops. Audio engine problem?
I'm using Sibelius 2019.7 on Windows and I had suspended updates while I tried to solve this problem.
Like you, I've monitored the cpu / ram usage and it doesn't appear to be a leak although I'm at the edge with respect to my 24 GB of memory. Network isn't over loaded either and I don't have wifi on this machine. Could be a thousand things including an older audio interface/driver etc. so I'm glad I can just restart the server to solve the problem for another week or so. I have to say it's pretty random for me.
I'm having a blast with Synchron / VE7 / Sibelius, so for now I can live with the system restart each week.
I hope restarting helps you get back to work until someone figures out what's going on.
Alain