red targa

I was driving up some mountain roads behind my friend's C4S with Lotec turbo. For those who are not familiar with Lotec, I think they are now called by another name - sorry, can't remember. The turbo charger is water-cooled interestingly. Doesn't seem to have much low end torque, because I can easily keep up with him with my Targa up until 150 km/hr or so.

Anyway, every time the C4S slows down, I hear this kind of popping sound. My friend says that he only notices it when he is coming down from 3000 or 4000 rpm to around 2000 rpm or so. It doesn't happen when rpm is going up.

He is waiting to have it checked by the garage, but can someone give some comments or suggestions about this.

Regards,