Startup peculiar behaviour

SInce quite some time I have startup phenomena. What is going on? What can I do about it.
1- When I startup quite often the Mac Mini chimes and apparently does not continue for ever with a black screen.
2- When I restart with Shift Control PR it realy startsup
3- During Apple the progress bar display the MacMini generates six loud ticks via the audio port
4- Several times the MacMini starts without showing the Apple Progress Bar during startup. After the chime the screen is black and the login screen is displayed.

Who can me explain that strange behaviour

Gerard Freriks

I can’t explain it but I have two things to try. First of all, run Apple Hardware Test:

Second, consider if the battery on the motherboard might be dead. That’s purely a guess, but it’s worth checking.

I did a test by restarting with the SHIFT pushed, and surprise, surprise, I heard NO sound via my speakers.
This suggests software (extentions) as the source of the pronlems.