Could be the timing belt slapping on its cover. When the tensioner starts to fail it's common for this to happen. The noise going away as rpms increase seems to be the key.

When was the last time you had the timng belt and tensioner replaced? You don't mention your Rodeo's mileage but if it's never been done and you're over 100K miles it could be that you're due and the timing belt is telling you so.


Steve

'97 Rodeo 5spd. 3.2L SOHC V6 4WD