engine noise

I own a 2000 taurus with 84,000 miles 3.0 litre engine, and well maintained. I have an engine noise or clicking coming from the passinger side of the vehicle. At first I though it was a pulley or belt tensioner, and had the dealer look. They could not find anything wrong? The noise increases as I increase speed especially under load, excelerating or going up a hill. I am wondering if I my have a lifter or pushrod problem? Any feedback is appreciated. Any suggestions on test I can perform?