I've got a noise that seems to be engine speed and load dependant.
Under partial throttle acceleration @ 22-2300 rpm I hear a noise from
under the hood that sounds like a plastic coach's whistle (little ball
bouncing inside the case) but somewhat muted. The noise is more of a
plastic clacking rather than a shrill whistle. The noise also seems
to be temperature dependant, more likely above 80C on the water temp.
The noise diminishes as rpm increases but is still there @3200 rpm and
74 mph. As the the car shifts through the gears, the noise comes and
goes as the rpm increase and then drops below 2300. I can give it
full throttle and the noise isn't apparent, and I can release throttle
at cruise and also make it disappear. At times when the engine is
cold and warming up on the highway at 65-70 mph the noise starts while
under a constant load as the water temp climbs >80. Any ideas?
My experience is that odd noises involve the air cleaner's rubber mounts
or that its metal support bracket's rear leg is broken from metal
fatigue. An easy check is to try to lift the air cleaner - if all is
well it won't lift.
At the aft end of the intake manifold is a "banjo fitting" (which
periodically ought to be cleaned) to which a clear plastic hose is
attached. That hose goes to an electric powered "switch over valve" on
the left firewall and then on to the injection pump.
The purpose is to transmit the turbo's boost pressure to the injection
pump so it "knows" to add extra fuel during acceleration. The electric
switchover valve is a safety valve to prevent a run away engine.
The turbo's boost pressure starts at about 2,000 rpm and rises to about
10 psi during full acceleration. The occurance of the noise is
correlated to the turbo's boost pressure.
I believe the switch over valve (or a hose to it that's split) is the
noise that you hear. The valve gets dirty and the rubber hoses to it can
split. The electric valve is actuated by a 16 psi sensor in the top rear
of the intake manifold - also check the sensor's electrical connection.
An easy test is to by pass the switch over valve - but not permanently.
The valve can be cleaned with carburetor cleaner or similar solvent.
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