Kirby Palm nous propose sur son site une collection de bruit de V12
An attempt to start an HE V12 with a bad ignition amplifier -- you hear it want to almost start, but it's just not getting good spark. This same engine will turn over just fine by swapping out the AB14 ignition amplifier box with a good one.
If you inadvertently disconnect the coolant temperature sensor (on the front of the B bank), or if it has gone "open circuit" (possibly after it has run warm), this is what starting will sound like. Note the subtle difference between this and the above sound -- there's no "almost fired" sound here, because there's no fuel being injected to be ignited. Note that you could get the same sound with no fuel and/or a dead fuel pump - but presumably you'd have excluded these possibilities first.
This ignition amplifier is marginal.
This is the same marginal ignition amplifier. Note that it seems to start and idle fine, but as throttle is applied, it stumbles. Notably, it runs nicer when the engine is unloaded (in neutral gear), but when you put it in gear and apply throttle (towards the end of the recording, after the 1 minute mark), the car doesn't really want to go. The recording has several "rev ups" - where I apply light throttle (only to about 2000 rpm or so), demonstrating how sometimes, but not always, rather than revving up, it stumbles. Note that this was recorded with the microphone right next to the wiper grille, whereas the above sounds were recorded with the mic on the roof of the passenger cabin, where it is obviously preferrable to capture from.
Tony Parran, owner of a 1984 XJ12, provided the audio for this sound. The racket given off by a starter with what his repair shop called "a broken brake assembly within the starter unit, used to decellerate the starter once it has been disengaged." Unfortunatley, there is no such thing - chances are, this is actually a bum Bendix.