99 Elantra Engine Noise

My 99 Elantra DOHC is up over 100,000 miles now and just recently started making a rattling noise, mostly noticeable when idle. It's always been a loud car but I think the noise is different now. Haven't noticed a change in performance.

I have a long commute everyday and was wondering if I should be concerned about the rattle in the engine. Fluids are topped off, battery is cleaned, but I don't know what I can do to quiet the engine.

Suggestions???