I have a rodeo with 83,000 miles which had a rattle from the engine on
reving. problem was traced to a bad fan clutch, unfortunately the fan
bracket broke the day I bought the replacement fan clutch from autozone.
The autozone replacement fan clutch was smaller and doesn't seem like it
fits like the original. It allows the fan to be further from the shroud and
seems like it is noiser than the original fan. It definately seems louder
than the original fan clutch. I am tempted to return the autozone fan clutch
and get another as this one doesn't seem to work as stated in the literature
that came with the replacement.(Stiff when cold and not noticablely stiffer
when hot.)

I have replaced the water pump, timing belt and a idler pulley which was
noisy. the engine runs great otherwise except for the loud roar at about
2000 rpm when shifting.

Any ideas!!?

 

I would replace the fan clutch. I have a Tundra that roars to when its first started up then goes away. It may be as you said to small.