Weird/annoying noise



I have an 09 Xterra with a 6 cylinder engine. It’s been running fine, except for a noise that only occurs exactly at 1800 rpm. It’s not too loud, almost like a rubbing/grinding from the engine compartment. Does not perfect performance at all, and barely noticeable with the radio playing. Noise does not occur in neutral, but does occur in any gear. I’ve been using a synthetic blend oil, and switching from conventional did not make a difference. Any thoughts would be appreciated. Thanks!