I just started using Serenity Free in Sonar 8.5 32 bit, on Windows 7, and experienced this problem which has been noted a few times before:
http://www.kvraudio.com/forum/viewtopic.php?t=330273 ("Serenity-Pro makes noise when loaded in Studio One")
http://www.kvraudio.com/forum/viewtopic.php?t=322805 ("Serenity-Pro makes noise when loaded in FLStudio")

Serenity is one of many synths in the project, and when I open the project, or when I add a new instance of Serenity to the project, there is a very loud burst of noise, which fades off after 10 seconds. (Not very 'Serene'! )

Has anybody worked out what causes this, or is there any chance of a fix?

have a look whether there is an option in the properties of Sonar for VST plugins to bypass audio when loading plugins.

I've had this happen with Serenity Pro and Ultraswamp and have not been able to open projects in Reaper and all work is lost. Seems random and other Vst's don't do this.

When will there be 64Bit HGF plugins?
The SE dev environment will take quite some time (too much!) to fully support creation of more sophisticated of 64Bit plugins as a lot of the required additional modules will not be available in 64Bit versions at least for some time if at all. Also this does imply that upgrading existing 32Bit plugins to 64Bit will hardly be possible. Personally I don't think full 64Bit support for some sophisticated HGF plugins will be available before late 2013 or even early 2014 with SE. Being far from happy with such prospects and also with the limitation 'Windows only' I'm working on a different solution to adress this. This will take at least till summer 2013 so meanwhile I'll be concentrating on related projects like doing soundsets for e.g. Waldorf Blofeld etc.