Some background first:

1992 940GL Non-Turbo, 200,000 miles.
Oil changes every 3,000. Oil looks good.
Did not have W917 filter, so used Purolater "Pure One" at last change.
Runs great, does not overheat.
No warning lights. Have not checked for codes.

The problem:

Just in the last week or so, I started experiencing a "clacking" noise (sounds like "lifter" or "tappet" noise)that is perfectly rythmic, and gets louder when the engine is under load, such as traveling up hill. Actually, when the engine is not under load, such as traveling on level surface, not accelerating, or downhill, I can't even hear it, even if the rpm is the same.

Example:

I drive 45 miles to work... nice, hilly country highway. Keeping it at about 2500 rpm with the cruise control, the rythmic clacking is not audible with little or no load on engine. As the car starts traveling up hill (and we have some good ones), the clacking gets louder relative to the load on the engine. If it's a steep hill, the clacking is loud.

One thing I'm sure of, it's not "pinging" (pre-detonation?)... I've experienced that in other vehicles in my time and I'm familiar with the sound and feel. If it were a problem in the valve train such as a bad lifter, etc., wouldn't the noise be there all the time, regardless of load?

Any ideas/suggestions are appreciated!

Thanks...

Steve