Buzzing noise when headlights on

My mom has a 1996 Chevy S-10. Sometimes there will be a loud buzzing noise coming from the dash area when the headlights are on. If you turn the headights are off, there is no buzz. Any ideas?

My 85 does it too.

I just thought that it was a headlight warning.

If the key is on the buzzing should go away

I know the warning goes off if you switch off the truck with the headlights still on; all vehicles should have that to prevent from running the battery down. :(