Alright here's what I have found to be a tough one to figure out. I have a '95 4dr 4.3 blazer and when I bought it and took it out on the highway on its first long trip I noticed that about 150 miles into the trip something was making somewhat of a moaning noise (or sounded almost like an old-school fire engine siren). This rarely happens but when it does the only way to get it to stop making the noise is to slow down to about 35-40 MPH. So if I am going 75 MPH it sounds mostly like air, but as i slow down and get closer and closer to 35-40 MPH the noise gets a lot louder and then suddenly stops. Everything seems ok. We figured it might have been the fan clutch so we put a new one in, and it still is making the noise. Keep in mind that this "noise" only happens every so often, like literally 3-6 times a year, but I'm at a loss for trying to figure out what it is. I've taken a mechanic friend of mine with me for a ride once after it did it and go figure, like always it wouldn't make noise...

any ideas?

 

Does it have 4wd and does it work. Its possible at some odd weather condition something in the 4wd system is engaging. The fact that it only does it a few times a year seems like it is something that the whether is effecting. Is it very wet or very dry or do you do any off roading before it happens? It could something like an unprotected wire getting grounded out when very wet or something like that. Does this only happen when you go on trips that are like 150 miles or are some of those few times a year on short trips?

 

The 4wd works perfectly, I myself have never had her off roading and i don't believe the previous owner did either. As for the weather conditions, it usually happens in the summer but it doesn't seem to have anything to due with the temperature, and I know its not moisture because it has never happened in the rain or after it has rained or been washed...

 

could be your wheel bearings. a friend had a s10 and his did a strange noise and it was that. hope that helps

 

sounds a lot like a problem I had. I used to commute 45 miles to work every day. At first it started to make this noise if I drove the whole way doing 72 or so. I found that if I went slower or shorter trips it didn't make the noise. eventually I was doing 68, 65 and you see where this is going.

Months of struggling to figure it out, later.....

I replaced my vacumm switch on the t-case.
It had gotten week over time and was partially engaging the front axle when the engine was under a constant load for a long time. The vacumm stabilized and eventually partially acctivated teh switch. It too would stop when you slowed down, change in engine vacumm. Simple test is to disconnect the vacumm hose from the activator under/next to the battery tray. you could pull it off the vacumm switch, however there are three hoses there, one provides vacuum and if oyu disconnect it you will mess up the vacumm to the rest of your truck.

Its about a $12 part and all of 2 minutes to change, most of that time drinking the beer while looking for awrench to fit.