Buzzing sound

I just bought a 99 e150 and you know that buzzing noise when you leave the keys in and open the door. Well when I start the van (with door closed) it makes 4-5 long buzzing sounds then stops. I tried putting the seat belt on but that doesn't work, it does this every time I start it.
 
 
Thanks
 
 
James

My 97 has a timer for the seatbelt buzzer.  Fastening the seatbelts dosen't change the timer.  Probably, yours is the same.   You get use to it.