Loud sound when i get my 1990 f-150 up to 50mph on the highway, i sounds very very loud, and it pretty hard to get it up there, afraid to go faster, cause the horrible loud sound

Yes I would get it checked before driving again..

If it sounds like a jet taking off then it's most likely some rush of air so where/how does air under pressure come from? Well, it comes from your exhaust but you said that's been replaced. Could be a cylinder head gasket in a engine but you would have felt a lose of power if that was the problem.
Is it a bearing that just sounds like an air sound? As I've suggested with other people...have someone follow you, then pull up along side of you on both sides and see if they can hear/isolate where the sound is coming from.

Sometimes when you're driving it's hard to hear exactly where the sound is coming from because well....you're driving!
So having two people in another car riding along side of you on the highway or unpopulated back road can hear it better.

Just remember to do it safely...you are on the road after all but if you hear it at 40...you should be able to hear it loud and clear at 55 or 65 or whatever the speed limit is.

Going too slow on the highway is dangerous too so obey the speed limits.

Sounds like a bad wheel bearing to me,with the car on a lift and in neutral..grab the wheels at 6 oclock and 12 oclock and see if there is any up and down or in and out play..if there is ,thats the culprit.Hope this helps

It sounds like an ignition problem. I need to know if you have scanned the computer for fault codes?

Could be the serpentine belt rubbing on something, but is probably a bearing problem in the belt idler pulley. However, it could be the power steering pump, AC compressor clutch or generator. As the water pump is at the opposite end of the engine, it's probably not that.

the drive band is going out it will stop running pretty soon

for 15 minutes no problem but fix it the soon you can if not you can burn the o2 sensors

check the tension of the power steering pump belt

It sounds like an idler pully needs replaced. The bearings are going bad. It is not hard to do this, but more than likely that is what it is. You should also replace the belt.

wonder if the catelatic converter packed it in