...key IN start position...

Hey Guys,
My engine and tranny have not been fired up since rebuild and reinstall.
Soon!
Yesterday, under the car with the key in the start position I could hear a very faint humming noise. I got a piece of rubber hose and used it like a stethoscope ...tracked the noise source to being real close to the tranny oil pan. With the hose against the pan, the faint hum turned into a high frequency ticking. Not loud at all. Goes away when the key is turned off.

I'm paranoid...so, wondering, any ideas what this noise is and if its normal or is it indicative of a problem.

Many thank-KK