I have a 90 Dodge Spirit that has a hum coming from the transmition.
The high pitch hum starts when I start the car up and continues while I drive and in park. The trans is working fine thou. Any idea where this is coming from.

Maybe its reminding you to change the oil and filter?

Do you mean the trans fluid and filter?

Yes transfluid and filter, also make sure its level is up.

If the fluids are all full check the Torque Converter... my spirit has a "hum" noise or a whin and its torque converter is gone

Could be power steering pump your hearing,, just a idea

If it is the power steering pump, just smell the fluid, if it smells burnt then that could be what it is, but if the sound is coming more from the transmission side id still say it was the torque converter... When you start the car, does the sound instantly start to hum, if so and your sure it's the transmission could still be torque converter... does the car have any pull, like does it move, if it does its probably not the torque converter but maybe ac compressor, power steering pump or could be alternator... but those aren't close to the transmission, first you have to track down where the noise is coming from inorder to find the solution.