I have an 08 Aspen with the 5.7L and about 10K miles. At cold startup I hear a very loud "rattling noise" that appears to come from the driver' side of the engine bay. (It sounds like someone hitting a tin can with a metal object very quickly). The rattle is RPM dependent (get's louder as RPM's increase) and occurs either in neutral or in drive (at 1400-1600 rpm very loud in drive until it dissipates). This started in the cold February months and persists even with the warm spring temps in May.

The trouble is that the "rattling" will last only for about 5 minutes then quickly goes away (engine not totally warmed up yet either)never to return for that day even upon repeated startupsafter letting the car/engine get "cold" again. It only occurs on the initial startup once per day,strangely for about 3-4 days in a row then it's gone for about 2-3 days. Intermittently occurs over several weeks then stops for a while. Very weird. Cold/hot weather seems to have no effect. A/Con or off has no effect (although the rattle sounds vagely similar to a bad AC clutch bearing). Turing the steering wheel has no effect. Doesn't appear to be an idler pulley. Exhaust is tight. Not sure if it's related to the exhaust or a hydraulic issue (PS pump, ABS pump, etc. that rattles until it pressures up?!)

I've left the Aspen overnight at the dealer (to simulatethe cold startup) 3 times and the rattle was documented only once. Again, the tech only has about 5 minutes to diagnose it so by the time he drives it from the lot onto the lift the rattle is gone. (Once the tech even left it on thelift overnight and it was quiet the next morning on startup -Murphy's Law again). Dealer has been patient/great but unable to find the cause as of today. Understandably they won't start replacing things (like the cat converter if the grate is loose) until they find a cause.

Any suggestions? I've read about a "flex joint" in the exhaust pipe just after the cat converter as one possible cause. Any other ideas/fixes?

UPDATE 5/13/08: After leaving the Aspen overnight again the determined tech heard the rattle faintly. He called Startech again and they told him to check the "flexplate". Tech pulled the trans and found 3 loose bolts on the flexplate (by the flywheel). Tightened the bolts, locktited them and now it's been quiet as a mouse for a week. It took a few visits to find it but I think it's fixed. Outstanding customer service from the service writer and technician at this PA dealership with their display of patience and determination to fix my issue. I think I owe them breakfast on my next visit.

 

Is it still quiet as a mouse with no drive issues?? This is great info as I fear I might be having something similar rearing it's head. We're not past my wife saying that it rattles under the hood occasionally yet. We usually go from there to complete mayhem quickly. Honey... there are a bunch of lights on in the truck... only for a few weeks... UPDATE ON MINE: just a burnt up idler bearing...