Valves tapping and clutch making noises

Okay I have a couple questions. I have an '02 Nissan Maxima. A couple months ago I started hearing a tapping sound coming from under the hood. We change the oil and oil filter ever 3000 miles or so. And I checked the oil and it says that its full. I was wonder what it could be. Also when my car sits in idle it makes a little bit of a noise and when I push the clutch in the noise goes away. We got a new clutch and flywheel almost 2 years ago and it cost a hefty penny to get it fixed. Do you think I need to fix it again?

When do you hear the tapping sounds? When it is first started? How long do you hear it? Is it loud or soft?

well it was kinda loud and it only happened when i was excellerating. but i drove it today and i didnt hear that noise it was different whenever i would shift gears it sounded like a bunch of balls rolling around a tin can.