When turning the key to IgnitionOn-position, there is
normally a 2-sec faint Sch-sound as of some mechanical movement
(eg some airvent-door shut by wirepull?). On MB A160 -99.
Question: What device cause the sch-sound?

My problem: Occasionally, the sch-sound is missing, and then the start fails!
Ie as the key is turned further, starter engages, but mixture? give no ignitions,
or only a few if one has stocked up some droplets by repeated ignitionkey On/Off.

By beginning from key-off again, the sch-sound has often re-appeared,
and then starter-engagement has always given start success, ie the
sch-sound comes from some essential start-preparation being done.

Gradually, my sound has become more rare, need 100 ignitionOn to appear once.
At IgnitionOn, the sch- and the electric fuel pump should start simultaneously:
Schhhhhhhhhhhh 2 sec -ends after 2 sec, then one can hear the longerlasting:
Zzzzzzzzzzzzzzzzzzzzzzzzz 4 sec -from fuel-pump, in action.

Complete Timeline: Removing batterylid, one can hear R-elays working ok, at least
2 R-Clicks at IgnOn (3 R-elays from right get warm ok=computer,pump,ESP).
At 1sec a faint double-click where fuel-line enters enginecompartment(valve?)
At 2sec a R-Click ends the Shhhh (rare to me, cant locate sh better than "engine").
At 4sec a faint click from fuelpump halting, at enginefront.
Turning IgnitionOff may after some secs cause 2-7clucks (fuel return,so pump ok?)
Most quarters I spray-clean the MAFS, with some apparent effect. Hasn't helped now.
(wish I could tmp. cut Mafs-signal from ICU as when they were separate units?)Thx