Whirring noise only when accelerating
 

About two weeks ago I noticed a whirring noise coming from under the truck towards the front end it sounds like. It's a 1999 4X4 4 door. It only occurs when accelerating. When you let off the gas there is about a second delay then it goes away. It does it from about 30 - 35 MPH and again at about 55 - 60 MPH. Also right before coming to a stop as it shifts down into 1st (I assume). My thoughts are either the transmission or front differential. I checked the transmission fluid and it was low so I added some, no change. I plan on a fluid an filter change as soon as I get some time. Any ideas from the experts out there?

could be the fan clutch. which is normal as all of mine has always done it which gets annoying. do you have 4wd?

Yes, it is 4wd.

Does it sound like the engine fan or a gear whine?

It's kind of hard to describe it is higher pitched at higher speeds. It sounds kind of like a high pitched buzz or hum.

Probably as suggested above, fan clutch. Normal operation for some reason on the Blazer...do not know why, but it is.
You can check the fan clutch: with engine OFF turn the fan by hand...should be a slight resistance.

I checked the fan by turning it by hand. There was some resistance but it wasn't constant. It ranged from a considerable amount to almost none depending on where in the revolution it was. Does this mean that it is bad?

Is the noise, vehicle speed related, or engine speed related?

edit: Those are typical speeds for a universal joint in the rear driveshaft to make noise or vibrate. Have you checked them?

It is vehicle speed related. Even in different gears it comes at the same speeds. I didn't think of that.

Power steering fluid low? Alternator?