If you have a strange tapping / rattle / vibration that seems to come from the motor area you may want to not drive the vehicle until you pull off the intake plenum and inspect the little throttle intake butterflies that do something (but not much, because removed mine and no difference). There are little screws that come loose and then the half-moon shaped butterfly and screws come off and get ingested into the motor. Mine ate one screw but was okay.

My 2003 Montero 6G75 3.8L had this issue at speeds usually over 65 MPH and usually only upon fast acceleration. When I'd let off the gas pedal, it would usually stop right away as it coasted. Sometimes it came and went but usually it stuck to this pattern.

I found the fix on a remote website from a dude oversees who ran Monteros for his touring business in the desert.

It's an easy fix. Remove entire intake. Turn upside down and look into intake paths. Unbolt the rod the butterflies connect to, remove the screws and butterflies, and put the rod back in.

I got the Montero cheap because the dude thought it was a bearing or rod, lol. I knew it wasn't that, but it did take a while to figure it out and I found only one or two posts on the internet about them, none on the main Montero websites.