Drive train is getting loud in town and on freeway, differential?

it sounds like your cv joint bolts are not tight . so you probably should get back under and double check all bolts .

sounds to me like your drive shaft is going and that you might have bad axle to

Auto or 5-speed?
Subarus have CV axles in front and back. And a drive shaft from the center differential of the transmission to the back differential.

Yes, it could be the drive shaft or axle. If you turn the car hard left or right do you hear any noises - usually a 'clicking' sound? This is likely a CV axle in front.

Check the fluid in the transmission (Automatic) (info is in the Owner's Manual) to make sure it's full and/or gear oil in the front differential.

If it's an Automatic transmission, the fluid may need changed - NOT FLUSHED - if it has more than 30K miles since last changed.
GL,
TD

This recently happened to my 98 Subaru Legacy Outback.
My problem is in the rear differential. May be the same for you.
When making a turn, the car sounds like a Star Wars T.I.E. Fighter. The outside wheel is not turning fast enough to keep up with the inside wheel or vice versa. AWD is disengaged and car is currently operating on Front WD. I was told this is fine to drive this way.

1. on my 1998 subaru forest, when i drive for long periods of time, i lose alot of oil leaking from the engine somewhere. itcoats the underside of the car and the back window. i have cheched for leaks but have found none. help

Just a thought, Check under bonnett on passenger side is a fuse that when installed dis-engages the rear drive. Remove the fuse to engage the rear drive. Sometimes people see the open fuse holder and fill the slot thinking it is missing. Truth is it is only for short towing. Good luck and have a great day.

You should have the drive axles checked. Also Wheel bearings. Are there any vibrations or pulling?

Carefully pry the cover from around the shifter, usually rear toward front easiest. Then you will see the light fixture. If you still don't see it and need to you can remove the 2 screws at the rear which release the cover from floor pan. then you can lift it to reach easier. Hope this helps and have a good day.

Have you crawled under the vehicle and personally inspected your exhaust line from manifold to tail pipe? The roaring sound usually occur when there is a hole in the exhaust line. I had it happen to my forester before. A hole half the size of a dime made my subaru sound like my old 74 chevy pickup.

The fuel consumption sounds about right for a turbo, perhaps a little high. Hwy consumption depends on how you drive. If you cruise at 100 kph, you should get around 8 L/ 100km. Round town, about 10 L / 100km.