My Raider (2.6l, 4-cyl) had a "drone" sound, but it was at transitional throttle, going from coasting to accelerating or vice versa. On mine, it was from having add-a-leafs in the spring pack. On a short wheel base Montero, the trucks with an automatic transmission have a short rear driveshaft. The add-a-leafs altered the pinion angle enough to make the u-joints angry and they made a noise as described above. Longer shackles would have been a better bet.

Don't know if this has anything to do with your rig (you should put your vehicle particulars in your signature so we know what you have), so take that as a 'for what it's worth.'

Jeep drivers are frequently surprised what these trucks can do...without spending big dollars replacing factory parts that you shouldn't have to replace, providing they were made tough in the first place. Monteros are made tough.

John B.