After burning our alternator I installed a local sourced Italian alt. This alt gave a weird signal to the tachometer and made the tachometer vibrate. The charging voltage was not stable when the starter battery was full, making the tacho vibrate. Since this was a temporary alt (50A) and I've got a new alt (100A) installed I didn't bother too much.
But with the 50A alt and the vibrating tacho I got the engine waring light + beep as well. There is a Voltage level and when the alt goes below that Voltage the warning is sound. With the engine in idle I got the warning every now and then, with higher revs I did not. The new alt gives a stable voltage when charging the starter battery so this fixed the warning for me