If it's the earlier W 124 series, there's that somewhat
irritating (maybe enjoyable to some) Alfa Romeo like exhaust sound
upon acceleration but overall the interior is quiet even when the
engine is revved to the limit, it can sound like an Audi 5-cylinder
petrol engine, which still is smoother & quieter running than
all the Japanese taxis here.
The W210 E 250 they gotten rid of the noisy exhaust note but the
running smoothness & quietness still better & quieter than
the 4 or 6 cylinder Japanese taxis here.
As for LPG taxis, they also have some Protons in Malaysia with
petrol & natural gas hybrid as taxis there. In noise, vibration
& harshness levels of course you can expect lower levels in
these areas compared to most (but not all) diesel taxis around but
problem i'll say is the relative fuel inefficiency of these engines
compared to diesels. Gas engines driving range is much lesser &
more frequent refueling required &/or you need much bigger
tanks so eat into lugguage or even passenger seating space &
hence comfort & utility level of the taxi is compromised !!
Also performance not automatically superior compared to the latest
& best diesel cars around.