91 Corolla clunking/rattling at idle

Subject 'bout says it all. Car has 163000 miles on it and has developed a metalic rattling sound at idle, in gear or not. I've never heard anything like it. Sound decreases with RPM. Sound is coming from engine. We've checked exhaust.

The sound is in beat with the engine, but has several impulses per beat. We've had a mechanic suggest bent valve and we're going to do a compression test, but the engine still runs real well.

My son went around the engine with a stethoscope and seemed to get the loudest noise from under the car on the bell housing. Could the torque converter be loose (its an automatic)?

My immediate impression was that the sound was coming from the other end of the engine, in the area of the belts and pulleys.

Any suggestions would be appreciated.

Dan