Laptop whines on battery power

[Mercutio]

I have an HP Powerbook 4550 sitting here that makes a loud, high-pitched whine when it's running off battery power. AFAIK, it's hardly ever used on battery, but I replaced the hard drive in it today and the first thing the user told me is that "it never used to do that" whereas I had assumed it was normal behavior while I was working on it.

Here's what I can say: it doesn't happen when the laptop is plugged in to AC power.
It does happen regardless of the state of software on the machine. If it's on and running on battery, it's making a high-pitched whine. Messing around with power saving controls in Windows or firmware or a Linux boot disk does not change anything.
It's not coming from the speaker and no volume controls change the volume or pitch of the tone.
The fan is in good working order and the problem is not isolated to it.
Changing the screen brightness or turning the screen off does not have any effect.
Putting the laptop CPU in different power states from sleep to 100% utilization, does not change the noise. If it's on, it's whining.
The laptop's battery is in decent health, around ~70% of original capacity.


My best guess is that there's a low-level component (a cap or something) that's in the process of crapping out, but I haven't been able to isolate the noise.
I also can't for the life of me think of a reason that replacing its hard drive would cause that sort of issue.

Clearly, this is a new one for me. Anybody have a suggestion or two for troubleshooting before I just scrap the goddamned thing?

 

[Tea]

Weird! I like your guess. Does the new drive draw more power than the old one? Is it practical to test with the drive removed? Or with a different temporary-fit drive?

 

[Mercutio]

The whine happens whether or not a drive is installed, from the moment the machine is powered on and running on battery.

 

[ddrueding]

Whine is typically from a coil rather than a cap. Does the quality of the whine vary with power draw? How about with charge level (voltage)? The good news is that if you can find the offending coil (if that is what it is), you can fix it with hot glue.

My theory would be that the ferrite rod in the coil was tacked in place (to stop the whine), but handling the unit caused the glue to fail. it has happened to me before on desktop boards.

 

[Bozo]

Can you put the old drive back in? Does the whine go away then?

 

[Mercutio]

Changing the installed drive has no effect.

I took the thing completely apart, something that's a lot more difficult than I'd expect from a business machine, and there aren't any surface-mount components that could be glued down or desoldered and replaced. The battery connection is on the mainboard and the internal AC connection is just a cable that runs to the jack. I've had AC adapters and LCD panels start humming but this is the first time I've run across this.

I can't find anyone selling a replacement motherboard for it and that appears to be the only component I could replace.

 

[mubs]

Whining on battery but not on mains indicates the whiner is sensitive to voltage. Have a spare new battery to test with?

 

[Mercutio]

Nope. And there's no surface mount components to replace or glue down.

I gave it back and told the user to just cope.

 

[ddrueding]

Indeed. Once you've verified that the fault isn't with any part you can fix, there is little point in figuring out what the problem is.

 

[Bozo]

Did you give the 'drop test'?

 

[timwhit]

I was going to suggest banging on it. That's what I would try.

 

[Stereodude]

Percussive maintenance. AKA: The Fonz method.

 

[LunarMist]

HP makes Powerbooks now? Damn. I assume it's out of warranty, but how close is it to be written off?