2009 Audi A4 loud engine startup sound & shake

Ya ur on the right track..also a loose ground can do that

Remove the air filter and squirt a bit starter fluid into the intake manifold , then immediately try to start the car if it starts , that is an indication there is no fuel going to the engine ,check the fuel system,if it does not start , you may have an ignition problem.

Could be spark plugs or a bad ignition coil (but the dealership should have checked the coils). I would replace the spark plugs and air filter (just to be safe) and see if the problem goes away. Both are pretty inexpensive and easy to do. Presumabely your dealership checked for fault codes in the engine and didn't find any?

You have no mentioned any testing being done, the engine control module needs to be tested for fault codes, this should be a passive test (KOEO) and an engine running test (KOER). This is called a OBD2 fault code test KOEO test and KOER test.

Hi sounds like its got a misfire on a cylinder best to get in on vag machine to get faults read out.

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....

sound like 1st gear might be slipping or stripped

probably a bit late to reply, but i had exactly same problem and now my engine has seized up, so please get it checked out immediately by Audi service center.

possibly a resister problem in the management system, this can be adjusted to correct the wrong idle, need to take to someone with the Audi management system software for correct adjustment to be made or replace box £150,, Ray

I had this as well, but you just have to wait a second. You can't remove it to quickly, the car takes a second to essentially unlatch the key.