Cold Startup Issues, Loud Rumble At Idle (<1000RPM)

[gpking]

Within the past two days the TB has started behaving oddly following a cold start-up.

Last night I cranked it and the gauges came on 1/2 brightness and the volt meter dipped below 9V for about 10 seconds, then the volt meter went back up above 14 like usual and the gauges returned to full brightness.
The headlights and all other electronics were not impacted by this.
I forget the name but I'm aware of the system check/test thing that happens and dims the lights, but usually that occurs 5 minutes after start-up, so it wasn't that.

To add to my concern, this morning I cranked it and the entire cabin was shuddering and there was a loud rumble coming from under the floor. It sounded like I was in a rice burner.
It felt like the vibration was coming from the transmission (the center console was visibly shaking), so I shifted between Park and Neutral a few times and it didn't seem to make any difference.
I popped the hood and all looked and sounded normal there, the noise seemed to only be inside.
I shifted into Drive and pulled forward and the noise seemed to keep pitch with the engine until I reached 1100RPM and it faded out.
Any RPM above 1100 sounded completely normal. In Neutral the noise was a little quieter, but it was still there.
I had no SES light either time.
I'm coming up on 70,000 miles, ATF hasn't been changed yet (100,000 recommended for non-fleet).
All fluid levels are good.
Ambient temperature was 18oF.
On start-up 5 hours later everything sounded normal again.

Here is a recording I took this morning on the driveway:
http://youtu.be/fB48Q-Ynypw

I texted my mechanic and based on just the noise I described he suggested:
1) Fan clutch failing
(I don't think so, it was only below 1000RPM and wasn't a roar)
or
2) Misfire/dropped cylinder
(again, no SES light and I didn't notice any loss of power)

I am clueless. I want to chalk it up to cold weather but I don't want to ignore anything that could become catastrophic.

 

[gpking]

I got a chance to do some research and thought bad motor mounts are a plausible explanation, I just don't think they should have failed this soon.
Also, it could be exhaust related.

I'll check it out once I get up and going today.

 

[sunliner]

This is on a 2003? I'd think the motor mounts could easily be suspect. Not sure if that explains the voltage drop though. Old vehicle, low miles though..hard to tell. good luck!

 

[gpking]

This is on a 2003? I'd think the motor mounts could easily be suspect. Not sure if that explains the voltage drop though. Old vehicle, low miles though..hard to tell. good luck!

Yes, 2003. The previous owner let it sit for a few years, but I've had it since 2013.

I cold started it about 20 minutes ago and there was no rumble. The voltage gauge was low at first but quickly returned to normal.
Visibly the engine wasn't vibrating abnormally. There was a faint ticking sound.
The fan has maybe 1/4" of lopsidedness to it.
I checked the ATF and it was a little low, so I added less than 1/4 qt.

All appears to be normal for now.

 

[kkeo211]

When my motor mounts went bad, the Envoy would shake badly when it warmed up a little bit, and in Drive while at a stop. Yours seemed a little different because it also happened when cold.

 

[TB2004]

Well I will tell you what happened to me, we had to remove the skid plate under the engine last summer to find a small antifreeze leak from the water pump. After puttingit all back together, we put the skid plate back on. By the way mine is not metal but a fibered plastic of some sort, with the door to get to the oil filter. Well when it is cold it makes a loud vibrating noise, if the k&n filter is on it is even louder. once it warms up its gone. When the weather clears up I will remove the skid plate and am sure the noise will be gone. But hope to get it back on without the vibration. Good luck.

 

[gpking]

Hmm, interesting.
It has been bitter cold on occasion but I haven't had the noise since.
The TB feels like it's running much happier overall now that the weather has warmed up a bit.

 

[littleblazer]

In the cold, the pcm delays charging or something for a few seconds, that is why the voltage gauge is low when you first start it, mine does this as well, hangs around 10 on the gauge for a few seconds then hops up to 14.