Audi A4 96' Beeps without a warning sign on the dashboard
 

Hello to all. I have an Audi A4 from 1996, and recently i have been hearing a beeping sound, by the drivers side, like the beep of when you don't have the security belt put on. The problem is that this this model doesn't have that feature, it doesn't indicate if the belt is on or off. The dashboard panel doesn't light up any sign of warning. What could this be? It beeps for 1 min and than it stops for 1min,2 or 25min or 1h etc and than it starts again.