Strange Engine Noise

Not quite sure how to describe this, but here goes...

When driving around town (so, 25-40 MPH range) the engine will occasionally make a strange sound. It kind of like it is dieseling or maybe a little like the engine is racing. There is no real noticeable change in power or speed. The tach doesn't change when it happens. And it never happens at hwy speeds.

any ideas???

Could be the clutch fan roaring. The cooling fan will do this sometimes when taking off from a start/idle when the engine is HOT. If the truck is roaring all the time the fan clutch is shot.
G/luck
Joel

Joel,

Hmm... any ideas on how to verify. It's not an "all the time" noise. In fact VERY intermittent.

Thanks,

Dean