I'm hoping those of you who know something might be able to diagnose this for me.

When my '94 LX Wagon's engine is around 1200 RPM there's a knocking sound under the rocker panel. When I'm holding the throttle open I can feel the engine shake slightly, like it is missing a bit. It goes away as long as it isn't in a narrow band around 1200 to 1500 RPM.

Without knowing any better, it seems like the exaust vavle(s) on a cylinder are floating at this RPM and the knocking I'm hearing is exhaust. But if this is the case, why do HIGHER RPMs make the problem go away?

Valves were adjusted and timing belt replaced a year and a half ago at 80,000 miles by a Honda dealer ($$$). Gas mileage is fine and unchanged, car accelerates fine and has 120,000 miles. Exhaust is noisy under a load, but there seems to be no single source for the exaust sound other than the area around the exhaust manifold.

This might be pertinent: I have a pretty bad pinging problem that I'm going to attempt to tackle by getting a tuneup. When the engine is warm I sometimes get really bad pinging under a light load. Downshifting the automatic usually helps. I can't get the 3 distributor bolts loosened to try to retard the timing. I'd rather not have to resort to a higher octane fuel.

Thanks in advance!