So my s4 t2 has been sitting for a while and I finally started it cold 2 times today. it makes a loud whine or whistle on first startup. It almost sounds like a really loud air pump. I don't have any smog equipment, no ps, no ac. Anyone else have this happen to them? The only things I could think of are the BAC or the turbo. I really hope its not inside the engine. My car has trouble idling because of tune. It is possible BAC also. I was by myself and I had to rev the engine to keep it running so I was not able to listen around the engine bay

 

sounds like a really classic big vac leak. like brake booster big

 

So I had a buddy help me listen again.
It wasn't the horn shorting out. It's coming from the front. I will disconnect my belt to rule out the water pump and alternator today.

 

Lol the horn...