560SEL High-Pitched Whine Post-Shutdown
 

Hello all,


A truly bizarre one – the 1986 560SEL has developed a intermittent, falling octave, high-pitched whine that starts maybe a minute after shut-down and lasts for 30-45 seconds or so.


It sounds for all the world like old-fashioned tube/CRT whine and it seems to be emanating from under the air cleaner.


Can't say that I have ever heard any system on any car ever make that sort of "electronic" sounding whine before.


Vacuum leak?


Fuel Pressure Regulator going bad?


???


BUT...


No smell of either fuel or evidence of a lean or rich mixture and the car idles exactly the same as before with no signs/symptoms of a vacuum leak.


Thanks in advance for your thoughts/experience!

With the engine off, after the noise stops, press on the brake pedal. If it is hard the power brake booster check valve is likely bad. Another source of a noise could be a failing vacuum check valve for a vacuum canister tied to the HVAC.

Thanks for the input.

The brake pedal is nice and "soft" after shut-down so no worries there.

Will check (no pun intended) the check valves nos. 0008000678 & 1268000078 associated with the HVAC/ACC "dashpot tank" no. 1238001219 anon.

Cheers!

UPDATE
 

Well it was the ACC vacuum check valve found just in front of the idle air valve that was to blame.

Ordered both check valves to make a clean sweep of things.

Now for the fun part - getting the replacement MB units, which look nothing like the originals, installed correctly...