Resonating sound from steering column & aftermarket A/C question

Great forums - learned a LOT about the Taco. Need some help with troubleshooting a problem.

It's a '97 Tacoma base, 2.4 w/AT on the column. I'm getting a resonating sound through the steering column when the truck is not moving - and it's most noticeable when the truck is cold. It almost sounds like something's rattling somewhere.

If I gently tug down or push up on the steering wheel it (mostly) goes away.

I checked the steering play and the front end bits and everything's tight.

At first I thought it might be the shifter assembly but the Tacoma uses a cable to change gears so there goes that theory.

Any ideas where to look next?

Also - I'm considering getting an aftermarket A/C kit - has anyone had experience installing one of these things? I know it needs to go in for the refrigerant charge but I was hoping I could install the kit myself.

Thanks for the help!