Double alternators for the win: 1993 Volvo B10M 1 of 2 alternators failed, Drove 50km home. 

It’s got the THD101GC with Schwitzer S3 turbo currently with 957,000km, It’s coupled to a ZF 4HP500 4 speed auto.

@@TheAussieBusman That humming sounds nice when you rev it. I think that's the sound of the timing gears and i think some late Leyland Tigers may have had the same engine fitted in Northern Ireland buses. Here in the UK some of our B10M's mostly had the THD102KF

The bit of a wine at idles the fan hub, But yeah when revved I think you may be right about the timing gears as it changes pitch pending in revs. A lot of our MK2’s and MK3’s had the THD101GC’s, With the earlier B59’s and B10M MK1’s having the THD100’s and the B10M MK4’s and B10BLE’s tended to have the later D10A’s so we had a good variety of the same base engine which was awesome.

@@TheAussieBusman We had Volvo Olympians with the D10A engine and they made a low pitch humm at idle and a bit higher when revved. It has to be the timing gears.