Warm-Up Period Exhaust Noise?

I have a problem that's been getting worse for about the last 3 months with our 2006 Elantra GLS automatic with only 30K miles. When I drive off with a cold engine/tansmission, during the first several miles of
city driving, and only when stopped at a light with the transmission in Drive and my foot on the brake, there's an apparent exhaust "knocking" sound emanating from forward of the gear selector. I've checked the exhaust system, and there doesn't seem to be any conflict with any chassis members beneath the car (my first thought). It's not an engine knock or piston slap, as I've already looked into this.
During the period of the noise progressively getting worse, I've also noticed a change in the exhaust sound, especially on acceleration. It sounds a bit "blatty," if you will - excuse the poor word choice - almost like a muffler in the process of failing. But, this shouldn't cause the knocking sound, as I would think a loose internal baffle would create a knocking sound. The knocking sound could perhaps also be construed as a pronounced rattle. For those who live in the snow belt, the sound is very similar to an exhaust tip on the back of the car knocking against ice build-up.
I've had a couple of thoughts, the first somewhat logical, and the second, out-of-the-box:
1). The flex joint,
2). The rear engine mount (mounted to the transmission I believe)???
I had the 48K or 4 year service performed at the local Hyundai dealer in December. The noise was present prior to taking it in, but they didn't hear it as the engine/transmission was still warm when then drove it into the shop for the work. I have noticed that the curb idle speed is a bit lower than prior to the service, and this lower idle speed exacerbates the knocking noise.
Any ideas?
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Quick edit to my post:
"...as I would think a loose internal baffle would create a knocking sound."
Should have read:
"... as I would think a loose internal baffle wouldn't create a knocking sound."
Sorry --
Don
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Hyundaitech or others, any input on this? I'm going to take another look tomorrow, and try to localize the source. I know what a bad flex joint sounds like on Dodge Caravan, but not on an Elantra, so I'm only guessing here.
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On 1/16/2010 1:52 PM, Don wrote:

I would almost say it sounds like the noise my 01 elantra makes when it's cold. But that is just the oil not reaching the top end yet, and that goes away within a 30 seconds. I would suggest leaving the car overnight with the dealer and tell them to leave it outside so it's nice and cold in the morning. That way they can hear it for themselves firsthand. Also you might try postin this over at www.elantraclub.com we have a few techs in the club that may be able to help.
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Thanks Ed. I will give that a try, before taking the car into the dealer.
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Sorry about being late to the party here. The flex pipe and motor mounts are potential causes, as would be perhaps a cracked exhaust manifold.
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