E320 Estate, S211, 2006 - high pitch noise from boot after battery change


New Member
Jan 26, 2021
North Hampshire
E320 Estate, S211, 2006, Auto
I had a battery warning on the dash and thought I'd changed the starter battery located in the boot.
The old battery recently showed well below 10v and when I cranked it, it dropped to about 7v and would not start - that's why I thought a new battery was in order.
I followed advice/procedure from youtube. I've plugged in a 'volt check gadget' plugged into my cigarette lighter to monitor the volts whilst the car is on.
The car started and I took it for a spin. I have two problems now...

(1) after a few miles drive, whilst driving the battery light came on again and I noticed that the charge came down from 14.1 to 12.0v and I had a few accessories on at the time.I switched off the accessories and it returned to 14.1v. I got home and left the car running and it came down again to 12.0v. Any idea of the problem? Hopefully it's not an alternator issue.

2) after I changed the battery, I can now hear a high pitch sound from near the DVD player and fusebox in the boot. It remains there whilst driving the car and also when car's ignition is off.
You can't hear the noise outside the car when doors are shut. Any idea of the problem and what I should do to clear it?

I thought the battery change would be straight-forward, obviously not.

Your help and advice is appreciated asap please.


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