G37 cold start rattling noise

[Archangele]

Hi All,
I have a 2014 Q60S, but it is the same thing as the G37 and I was wondering if anyone
with either the G37 or Q60 has this issue. When I start my car after sitting for several hours
on a cool day, their is a rattling sound that lasts for about 20 seconds, then the engine
slows down slightly and the noise goes away. I can rev the engine back up to the start
up revs and it will no longer rattle. This rattle is different then that timing chain rattle
in my older 2004 350Z. It sounds more like the exhaust has a leak or something. I had
listened to another Q60 of the same year and it did this as well. My car has only 9300
miles on it. The noise reminds me of the old American V8's of the late 60's and 70's
which would rattle because of this exhaust valve that would close and cause exhaust gas
to go through a passage in the intake manifold until the engine warmed up, then it would open
and the thing would be dead quiet. Does this VQ37 engine employ some thing like this
to possibly heat up the converters, or some part of the engine block? The car only does
it when started after it is completely cold. I just would like to know if this is normal
or if I have something to worry about.

[04pathse]

maybe just a loose heat shield on the catalytic converter/exhaust system

[Archangele]

The thing that is strange is that it makes this noise for about 15 - 30 seconds then it becomes completely quiet and never even hints at this rattling sound until it is shut off and let sit until completely cold. I'm going to roll it up on
a set of ramps and let it sit over night and see if it might be something just bouncing around like a heat shield.
But on a Honda I had, which had a similar rattle, it never went away and it was the front heat shield. In this car, the noise starts only when the car has sat for several hours on a cool day. Also, it only lasts 15 - 30 seconds and then
completely goes away. It almost sounds like something has to warm up and then some valve is closing. It
quiets down that fast. It reminds me of a 70's car with that old air pump system that would growl until the
car was warmed up, then the pump and plumbing would become quiet. I noticed this on another 2014
Q60S I checked out. Mine has 9300 miles on it and the other Q60S had 34K miles on it. Both make
this peculiar rattling sound when started after sitting for several hours and being completely cold.
To be frank, it sounds almost like an exhaust leak or bad exhaust manifold gasket but seems to be coming from under the car about even with the transmission. My Q60S is a manual while the other was an automatic so
I doubt it is the transmission and in both cases, the transmissions work fine as do the engines. Anyway, I am
going to check all the heat shields like you said. Maybe when they get warm they tighten up just enough to shut up.
I do know that the converters and pipes will get burning hot after less then a minute.