Rebooting Has No Effect. AVG Doesn't Find Anything

I have Vista Home Premium 64 bit. It has recently started going ding from time to time for no apparent reason. There is no dialog that pops up, and I haven't found any activity that alters the frequency of occurence, however the occurances are extremely erratic on one occasion there were several so close together they overlapped, I might get two or more in a five minute period or none for hours. I think they are becoming more frequent, but that could just be that I notice them more. Rebooting has no effect. AVG doesn't find anything.

I have found nothing that tells me why it is doing it - there are no entries in the system events that correspond to the dings. The only thing I have been able to establish is that taskeng.exe does access the file "C:WindowsMediaWindows Ding.wav" at the same time. This instance of taskeng is running under the logged in user credentials. Looking in Task Scheduler the task status doesn't change after a ding - indicating that an existing task, or the engine itself, is responsible.The list of running tasks shows the following MsCtfMonitor(Hidden, runs at user logon) SystemSoundService (runs at user logon)SystemTask (runs at system start)TMM (runs at user logon)UserTask (runs at user logon)

The last entry for these under history is "task started", except for SystemTask, and UserTask, which every 8 hours are rerun so show additional start and stop entries. The only errors are that UserTask fails to stop, and is restarted, when the system is shutting down, and SystemStart fails when the system starts due to a lack of a network connection, but does start shortly thereafter.ra.com/mail/