high frequent electronic noise in enginebay, once car is OFF

[LLantrisant]

since a few days i have recognized an electronic noise comming from the engienbay.
plenum /alternator area.

its a very soft/quiet High-frequency whistling, all the time when ignition is OFF.
once ignition is ON or engine has been started its not present anymore and returns directly when enigne is switched OFF.

the only way i can stop this sound is to disconnect the Battery (my car has a main power switch installed, cutting the Minus-side of the Battery)

since I am very sensitive to noises, this would have caught my attention already in the past. so i´m quite certain that this noise is new.

any ideas?

[FarmyardPants]

Have you had the alternator out? If so, I would check that it’s wired up correctly. Otherwise it could be a failing rectifier/diode pack.

Is there much current flowing across the battery terminals?

Also, what’s the dash battery light up to? Does it come on with ignition and go off when running?

[LLantrisant]

i have nothing done to the engine or inside enginebay....the aerea were i assume the "noise" is just estimated....somewhere around the plenum.....i have not measured yet, any power (ampere) drawn by the car.

Battery light reacts normal...so dashboard does not shwow any anomalities in terms of warning lights

[Imran999]

Get a 2’ length of hose, put one side to your ear and move the other end around the engine bay to locate exactly where the sound is coming from.
A length of hose is a critical piece of equipment for all sorts of classic car jobs.

[ridds]

I suspect this is related to your other electrical conundrums related to the fan control.

[Stunned Monkey]

Check the coolant expansion cap. Mine makes a quiet whining too.

[LLantrisant]

Stunned Monkey said:

Check the coolant expansion cap. Mine makes a quiet whining too.

this can be excluded, even without testing, as the noise is really "electric" and it also happens when the car is cold.

immediatly when the battery cut-off switch has been turnd to "contact" ,the noise starts.

[LLantrisant]

ridds said:

I suspect this is related to your other electrical conundrums related to the fan control.

on 1st view it looks like.....but no real "physical" modifications have been done to the electric-circuit for my fan-anamaities...i just "investigated" by measuring and testing.

and the "noise" started suddenly...even days later after my last "investigation-day" for the fan´s.

[Polly Grigora]

Alarm siren?

[LLantrisant]

ridds said:

I suspect this is related to your other electrical conundrums related to the fan control.

you are right....i totaly forgot about a slight modification i have done to my fan´s
after i reversed this modification, the noise has gone.

thanks for your help...and sorry for the confusion.

Edited by LLantrisant on Wednesday 26th July 17:19