I have AUDI A3 1.6 2001mod with 01M autom.transm.and while i drive in the highway ,suddenly makes a hard noise (bang) and seems to engage a very low speed and brakes for a second. No errors detected o

I don't think you will know whats going on until someone takes the transfer case apart. It could be the shifter stuck in mid position and it is not completely in either position.

READ THIS (happy ending so far...):
audi a4 2L turbo quattro, manual transmission with 98,000 KM. Perfect conditions, no oil leaks whatsoever.

Left the car off for two weeks in very cold weather (-10C down to -25C). When I restarted, it started immediately but with heavy rattling noise due to camshaft adjuster not getting enough oil pressure. After 3 minutes of warming up (outside temp.: -12C) at very low rpm (below 2,000rpm), I start moving the car. Still hearing heavy rattle noise. As soon as I pass 2,000 rpm, I get a scary red warning, low oil pressure. Turn off the engine immediately. In 1 minute, I re-start the car and the engine lights check ok, so I drive in first gear super slow (1,200rpm) hearing the same rattling noise. After another minute, finally the noise all the sudden disappears and the car starts running just fine.

Called Audi dealer and they told me that in extreme weather conditions, I should warm the engine for 10-15 minutes if necessary before start moving.
Take away: luckily, I didn't screw up the engine since I was using it at extremely low rpm (easy to do with clutch and manual transmission) until the oil reached full pressure. I am pretty sure now that the problem was leaving the car off for such a long time in freezing weather. All the oil flows back to the bottom, and when I restarted it, the cold oil with high viscosity took a long time to reach all the parts of the engine, specially the camshaft adjusters that are actuated by using oil pressure. Next day, the car started at -9C perfectly without making any noise (as usual). Basically, the longer you leave your audi in cold weather, the longer you might hear the rattling noise until the oil reach full pressure inside the engine. My car took 6 minutes to fix it after couple of weeks in cold weather. This makes sense to me; in fact, in the past I only heard the rattling noise after leaving the car off for a couple of days in the winter.
So far, this has had an happy ending....

the right outer tierodending needs replaced.

its either low on fluid or the clutches & bands are worn or need adjusting!
DONT REV IT!
get itserviced QUICK SMART! & check for leaks!

Is the coil spring binding from a bad strut mount bearing?

hHw many miles on trans?? Do you have a chack engine light?? does the car mark it's spot whever you park?? ECU and TCU are two seperate computers and (I believe both) are located under the black cowl just fwd of the windshield

sound like rod and main bearings failure.

Check your RPM sensor.  Mine did virtually the same thing.  It was NOT the fuel pump like Audi dealer said-after I repaired it.  

i would have this fix asap ,could cause internal transm. damage!

sounds as if you may be looking in the wrong spot. If different gearing is drastically changing the symptoms you may want to check the trans. or torque converter. sudden pop with engine rpm increase and lack of power sounds like possible band breakage in transmission or impellor breakage in torque converter.