After my '12 LT 5.3l Suburban sits for more than 4-5 days when I start there is a loud metallic clunk under the engine area. One clunk, car starts fine and runs fine, no more clunking while driving. If I stop and start during the day there is no further clunking sound on start up. If I drive it every day there is no clunk on starting, just when it sits for a few days does this happen. Battery is fully charged, alt. is fine. Is there something with the starter that might be the issue or anyone have an idea what it is? TIA

Daniel