Pop bottle noise???

[Chuck A]

We drive a 2003 V-70 non turbo, front wheel drive. At 1600 to 2000 RPM and between 45 to 60 there is the strangest intermitant noise from the front of the car. It sounds just like blowing over the top of a pop bottle! It doesn't happen every time you drive the car, just once and a while. My local Volvo mechanic thinks it could be the flame trap, but the noise persists even with the dipstick partly out which normally makes a plugged flame trap stop whistling. Anyone have any thoughts??? Thanks in advance...