Constant Key-in-the-ignition-type sound after changing bulb in gauge panel

I just replaced a bulb in my gauge panel, put everything back together, re-hooked my negative cable to the battery, and got this high-pitched humming sound (kind of like the key is in the ignition) which won't stop unless you actually start the car. Any ideas what can be causing this? Totally stumped. Thanks!

 

Welcome aboard new member

 

Thank you!

 

Welcome to TW!

 

Thanks!

 

So guys...after some deep studying, I discovered the solution: you close the f*$%!ing door.

I swear, I'm an absolute idiot.

 

I was actually coming in here to suggest that! Good sleuthing OP and welcome!