I have a 91 Pathfinder SE 5spd, and it makes a noise from the engine compartment that could be described as a howling, whirring or wwhhhooooo'ing. The vehicle has a new timing belt and water pump. It does not have a/c. This noise starts happening after the engine warms up, and takes longer to show when it is colder. It is not present while the engine is idling. It is especially loud during engine braking. Most of the time it is in synch with the engine rpm's, but sometimes goes away by reducing in frequency, independent of engine rpm's. I suspect something to do with the fan. It does not relate to what gear I'm in.
A few years ago, someone posted the following about their '92 Pathfinder:
("The howling starts at 1700 rpm's then goes away at around 37-4000 rpm's. It definitely sounds like it is coming from the fan but the weird thing is how it will cycle with the rpm's. I have replaced the fan clutch and water pump but no luck. Sometimes if I Rev it to 5000 it will go away till I turn off the motor but when I start it up, the howler is back. It is really loud and really annoying any help would be great!
One more thing, the sound will also come back as the motor slows back down, so as the engine noise is going down from being reved up all of a sudden this howling fan will get really loud and then it will become quiet when it reaches about 1700 rpms")
This sounds very much like our problem. There was no response to that post. I've heard of other instances where people have replaced the fan clutch to solve this problem, but it did not go away. Obviously I'm hoping someone knows what's going on, rather than throwing parts at it, or paying a garage to guess about it.
thanks