hi,

I have an '96 850 Turbo and I have the same symtoms.

It happens to me more often when the turbo kicks in or "kicks out". But
I've noticed today that it also happened when the turbo was not
"kicking" in or out. Like you it happens every 5-10 secs interval lasts
for a second - but I've noticed that as things cooled down in the car,
the noise would go away or has a lomger interval between "manifestations" :-)

I went to the dealer this week and explained the problem and the service
reps agreed with each other that it's just the turbo drawing air. I am hoping someone can concurn with the Volvo service guys on this.

Also went to a garage to get inquire about a r134a recharge, the "air con
expert" measured the temperature of the air coming out of my vent ( he
stuck a thermometer, that looked like a meat thermomether into the vents
on top of the radio) The temperature was 10C on a day where it was more then
30C outside. He said that I didn't need a recharge.

Hopefully my experience could help you.

Lee