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 ...

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... Kerry

126 tailed by a 203, 129 leading the pack.