What is that roaring noise?
 

2002 Chevy Blazer zxr 4 wheel drive 156,000 miles

A month ago the 4 wheel drive stopped working, push buttons for 4 whele drive where blinking and wouldn't go into 4 wheel drive. Took the transfer case shift motor off and replaced with a new one then found out it was just a vaccuum hose loose, replaced the old transfer case shirt motor and returned the new one. 4 wheel drive started working, lights stopped blinking.
THEN noticed that when sitting in the morning running it sounds rough. Paid not attention during the week. Weekend drove to town and sitting at gas station there is a noise that sounded similar to a far away lawn mower running, souonds like it is coming from under front. Drove a few feet and it sounded as if it was in 4 wheel drive( it wasn't). Drove home and it still sounded as if it was in 4 low( it wasn't). Hubs thinks it was coming from the transmission. Sounds like a roaring noise. Can't really tell since there is alot of noise coming from under the blazer to begin with. Also noticed slight squeaking coming from alternator. The noises come and go. I drove it for an hour half and it did it twice for about 5 minutes each time.
I am at a toss up between transmission problems or the alternator bearings.

Not sure what to do as I can't afford to put alot of cash into it and am looking to trade it in to begin with for a 4 door vehicle.
Any ideas?

Most of the time I look problems up on the internet but since it could be anything I really can't narrow it down. Other than this I have had it for 6 year with no real major mechanical problems.

I hate to trade in on spur of the moment but I really need a dependable car and can't chance if the transmission is going with 2 small kids and no other vehicle for it to break down and me not having anything!

Also possiblity of something messing up when hubs changed the transfer case shift motor out???

does the noise every go away? get louder with speed?

It does go away. It can run for an hour without any noise and then I will hear it for 5 minutes or it might happen when I have it sitting and running for 10 minutes. I noticed to day that when it was sitting in park and I revved it that the noise speed up so that made me think maybe it wasn't the transmission since I was sitting still in park.

is it roaring when stopped?

take a look for the rad fan clutch locking up