Thanks for the alternator suggestion, I'll try it over the weekend. I've attempted to pinpoint the location of the sound - it comes from the top frontal part of the engine, so as you said, it is very unlikely that the alternator is the problem.
The rattle is re-recorded
without the beeping.
How difficult is to replace the camshaft adjusters? I can only find the instruction on how to replace the camshafts in my manual. If I don't have to remove the camshafts, but just get to them by removing the valve cover and the front cover of the cylinder block I might go for it. I know that the camshafts must be locked by a special tool, any other special tools that I will need?
Can I see the adjusters if I just remove the valve cover and do the inspection?