Wierd Noise

[steve-o-]

hey guys,

i have a gts but there is a weird noise when i drive it like it comes on around 3500 revs but it is like a blow-off valve going off. it is pretty loud because i can hear it from inside but it only goes off in the first gear part of an auto. it does not happen all the time but happens about once every 30 drives. i have no idea what it is and dont want to go to the machanic and tell him bout it and look stupid.

thanx