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
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.