Lexus GX 2004-09 :: 2007 GX470 - Intermittent Engine Whirring / Zinging Sound

2007 GX470 120,000 miles. Great vehicle and no problems until now. Seems I have an intermittent engine whine/whirring/zinging sound. I say intermittent because sometimes it there and other times its gone. It is fairly loud. Almost like an extremely loud alternator whine, only this sound is coming from the area near the ac compressor. There is some plumbing which runs just along the left side of the compressor pump and when I grab those lines and the little aluminum cannister in that area, I can feel the vibration in that system, big time. No vibration from the compressor pump though. Not sure what those lines are, but whatever they connect to is probably the culprit.

Its odd because the noise comes and goes. It doesn't fade in/out. It's either full blast loud, or totally gone. Once it's gone, it doesn't come back for a few days. Then one morning i go out and start the truck, and there it is again! Usually lasts for about a day, then it goes away for a week or so. After a week goes by, there it is again! Really odd. I had my mechanic look at it, but it wasn't making any noise that day (as luck would have it). He told me it was an awesome vehicle. Of course I agreed, but that didn't diagnose the problem.