I have searched this forum for ideas and cannot find anything that is exactly what is going on with my truck. 1998 Blazer...started roaring suddenly going down the interstate (so loud I thought it was a motorcycle going around us). No lights come on nor do any of the gauges look suspect. It lessens as I slow down and when I come to a stop at a red light, it goes away completely.Going35 mph, tried putting it in neutral, roar is still there, BUT it does go away when I push the 4WD High button. What do you think?

Thanks so much.

 

The first thought that comes to mind is that you're partially engaged in 4wd and the front axles are trying to engage, so when you select 4wd the axles engage fully and the noise is gone.
Can you decribe the "roaring" better or pinpoint a location? Does it sound like it's coming from the front axle (left or right) or the front diff?

 

The roaring sounds like a loud engine...we were trying to decide for a minute or so if it was us or someone close to us racing their engine...like a motorcycle. It does sound like it is coming from the right. It happened suddenly while driving 60 mph. There was no pop or anything like that...just the loud roar.

 

My old Toyota pickup used to do that too. Seemed to be coming from the wheel area, not the transfer case. It did it more frequently when it was colder out. Never figured out what it was...

 

Are you saying your engine was roaring?

Was this an engine sound at high rpms?
Or was this a drivetrain sound your trying to figure out?

I guess I am a little unclear what you mean by roar?

 

By the sound descripion...I'm thinking...Cooling fan, if the cooling fan clutch locks up the fan can make an awfully loud roar.