6000 LE motor noise

6000 LE 3.1 Ltr. 155k miles. Just recently this this has been making a louder than usual sound while at idle. Seems to sound kinda, not a ruff idle but a like a sound when your low on oil. Maybe a little like marbels sound. Oil pump problem? If so.....how can I check to make sure the oil pump is working. Any other ideas? Other than that sound from the engine, it runs great. This sound just came about one day too. Thanks for the ideas.

Got it. It was the water pump. Thanks for the help.