Contour Enthusiasts Group Archives Forums General Troubleshooting Noise when accelerating

My '99 ATX V6 makes a bit of a rattling noise under heavy acceleration or acceleration uphill. I notice it a lot on the highway, and it hasn't affected performance or fuel economy. Putting in a higher octane gas didn't help, and there's no CEL. I cleaned the intakes (on-car method), replaced spark plugs and wires, cleaned the MAF and air filter. I'm sure that the noise can't be a sign of anything good, but it hasn't done anything bad yet. Any ideas?

Any other symptoms? High idle, rough idle? Does it only make the noise at lower RPM or do you hear it through the whole RPM band?

I would suspect:

Air leak & running lean
or
bent/loose heat sheild rattling a bit.

No problem idling... the car's running fine and doesn't make the noise at lower RPM. It happens at 3k+ RPM, but I don't hear it at any RPMs lower than that.

Had snow there lately? Might be that heat sheild I suggested.

No, but I was living in CO when I first noticed it. It's been a year since then, and it's gotten more noticeable. One of my friends described it as a "flapping" sound, so I'll check for that heat shield next time I have it up on the lift. I just assumed it was something bad in the engine, since the noise stops when I hold it at a certain speed.