603 (350) Frontal Motor Noise?

[ezrider]

1991 350SDL. Original owner, 149K miles. Has been a wonderful car.

At idle there is a moderate/loud hammering noise in/on the front of the motor that goes away after a high speed run but returns hours later after running around town. It is not valve lifters, it's a heavier sound. The noise has been around a good while, it's not getting worse.

Is there anything with the timing chain that relates to these symptoms? If it was the sepentine belt shock wouldn't the noise be there all the time at idle?

Thanks for any ideas.