noise from Ti only during battery use

[Don]

Greetings...

I've recently noticed that when I operate my Ti 400 on battery power, I get a high
pitched noise which is electrical (versus the sound of, say, a drive going bad). It
gets louder the longer the 'book runs and stops as soon as I plug in the power
adapter.

Anyone know what this might be and if it means I am on the edge of any kind of serious
trouble?

TIA,

Don