Is there a problem in my engine?

97 Honda Civic DX 2-Dr (Coupe) - Manual Transmission

My car makes a low clicking sound when it climbs a gentle incline (usually when I'm on 3rd/4th gear). The sound does not come when I climb a steep incline (2nd gear).

The sound is a very inconsistent, very low sound (similar to a penny clicking on a glass). I can hear it only when I listen to it carefully. But, I'm pretty positive its coming from under the hood.

I usually use 87 octane gas. The sound SURPRISINGLY goes away when I use a 89 octane gas.

1) Can someone tell me whether there is a permenent solution for this problem?
2) Could this be engine knocking?
3) Will I hurt the engine if I continue to run it under 87 octane gas?
4) Will the engine be okay if I run it on 89 octane gas?

your answer is highly appreciated.

Thank You,
Joe