Any ideas on this one?

I have a 99 GTP (64,000 mi). Last night for the first time I noticed a creaking/metallic groaning sound under fairly stiff acceleration from a stop. Doesn't seem to be the tranny, no problems shifting. Was wondering if it could be exhaust-related? Pulled into the driveway, put it in park and revved the engine--no sound. Put it in drive and accelerated while holding my foot on the brake and the sound came back. Did the same in reverse. Never noticed before last night (incidentally it got down to 15*). Could the cold temps have done something?

Have you checked the strut mounts on the engine?

You can have a friend torque-brake the car while watching the engine to try and find out if the strut mounts are making the noise.