1999 BMW 750il with a 5.4 12 cylinder. I did a tune up on it and now after the vehicle warms up it's making a ungodly howling noise down by the torque convertor area. But when you let the motor cool down and start it back up it goes away until it warms up again. What can this be? Is it something I did or pure coincidence?

You might check for an air plenum problem...

so it might have a vacuum leak in the plenum? I'll definitely check that, but why would the noise be coming down by the torque convertor?

could be the PCV valves on the back end of the plenum
(one on each side) it would make a sound like you described. to check this use a slack tube manometer at the crankcase, more than 6" of difference from side to side and that's where to start. repost if more help is needed