reason why air bag light may come on in 03 audi a4 1.8t fwd after front end work?

Hi, so a little while ago i had to do some work in the front of my 03 a4 1.8t 5speed fwd. had to replace the coolant flange and did a few other things for preventative maintenance. timing belt, water pump ect. but shortly after getting the car back together my air bag light came on. so I was hoping to see if anybody else has had this issue? or if its just me. and also im hearing this noise coming from the front of the car. drivers side im thinking. but its like a whistling sound at a certain rpm. and its mostly only in 4th gear from around 3700 to 4500 rpm. any ideas as to what this may be? any help would be greatly appreciated.