I know how annoying noises can be.
Here's my guess, while we wait for the experts:
Idling: I can't really define what is "low pitched and very quiet" to really be the problem. Could you describe more in detail?
Air Conditioner On: Try the compressor. Turn the a/c off, and the noise goes off.
Front driver's side suspension: check the tie-rods, suspension or the brakes ...
126 tailed by a 203, 129 leading the pack.