My bonneville is making a metallic rattle sound at idle. Sounds like someone is holding a piece of metal against one of the pulleys. I just had the catalytic convertor replaced, so I know its not that. Also, since this sound has come up, the car is starting hard when cold. I know both of these things are hard to diagnose without hearing it in person, but I know that these cars usually need the timing chain replaced around 150k miles, I am just wondering if that could be what is wrong, or if I should look for something simpler.

I just remembered, the car also makes the sound inbetween shifts, it seems to be whenever the engine doesn't have a load on it. Hopefully I will have a better description this weekend when I get a chance to look at it in the daylight. Thanks!

Just a couple of ideas... It could be a water pump, those can sound like a kind of grinding rattle. It could be the lifters, sometimes those rattle.

I'm not sure what it means that it comes more when your car shifts. That stumps me. Hopefully someone else knows better than I do.

The Timing Chain should be okay, if you only have 70k miles, that isn't your problem. Lifters.. well, I don't think those make a metal on metal noise unless the rockers are so lose they can just jump around [highly unlikely]. To me, the most likely thing is the Water Pump'* bearings have failed.. try to get under the hood, and try to follow the sound. If it is coming from the accessory belt, it is probably the WP.


-justin

Easy check: pull the serpentine belt. Start engine (OK for 20-30 seconds at least if cool when start). If sound is gone, it is in the accessory drive system. Don't forget to check the crank balancer/pully for a cracked rubber center.

Somewhere on the accessory belt makes sense, I will give that a try on Saturday. I think whatever it is makes the sound when shifting because the engine is slowing down causing slack somewhere, like the accessory belt. Kind of like the accessories are "coasting" until the engine is at its new speed. I don't know, sounds like a good explanation to me.