F355 humming sound from rear with tender hooked up

I guess 5~10 seconds doesn't give you enough time to pinpoint the location of the humming. Have you hooked up the tender to the engine compartment terminals? Could it be the tender itself making the noises?

 

Completely normal. It is the idle control valve. Drove me nuts when I first got my car as well.

It looks like this:

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Why would the idle air controller be humming with the engine off?

On the hot side? On the cold side, there is the alarm ECU? Is the car a 2.7 or 5.2?

Do you have any aftermarket stuff fitted? Exhaust Bypass controller wired up to the battery? Aftermarket stereo system picking up interference?

 

No idea on that. But that buzz is most likely from that valve as I had the same thing after shutting the engine off. It lasted just long enough for me to run to the back and find it. I actually replaced the valve 10 years ago thinking it was broken....same thing happened with a new one.

Is voltage getting to it when the maintainer energizes to charge the battery and it buzzes briefly?

Do you have the battery disconnect off? I always do that so no voltage walks back to the electronics.

Ian, any idea if there is a delay anywhere in the electronics from the time you switch the key off until all voltage is dropped?

 

IAC power comes from a relay which is energised by the respective Motronics ECU (if there are two ECUs). The 2.7 Motronic car has two idle air controllers, the 5.2 Motronic car has one controller. Normally you'd need the ignition on before the ECU will energise this relay. However...

The behaviour you observed would suggest that the ECU has some kind of power circuit with big capacitors and takes a while to remove power from the relay. I guess you could prove it was an IAC by simply pulling the plug/s, but why would the relay or ECU power up after two or three days? The ECU has battery power on it at all times, but you still need to turn the key to get the ECU to activate the relay. Hope it's not a faulty ECU.

Curiouser and curiouser...

 

Idle Air Controller.

No. Using the disconnect knob, you may prevent the battery charging (especially if you have the tender hooked up to the terminals in the engine bay.

Or another possibility. The battery power going to the ECU allows the ECU to power down it's relay control circuits over a certain time period. You wouldn't want permanent power on the control circuits as this would drain the battery.

 

The valves in the fuel evaporation control system can also vibrate/hum/wheeze for a while. Still could be something else as the others indicated (I don't know 355 well), but you can easily rule this in, or out, by just unscrewing the gas cap to quickly vent out the residual pressure in the gas tank (and/or leave the gas cap loose so pressure never builds "days after it's been driven"). Your remark -- "This keeps repeating randomly days after it’s been driven" -- seems strange for an electrical item. Just a thought...

 

The two-way valve (the drivers side of the car) sounds like a kazoo or a buzzing noise:
(the red "thing")
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