2002 340 DA At idle, and up to 1200 RPM, I hear a air hissing/suction noise at 1-3 second intervals. Loud enough to be heard with the hatch closed, very noticeable with the hatch open. The “breathing” rate increases from idle up to about 1200 RPM and then goes away. Becomes less noticeable as engine warms up, but doesn’t go away completely. It’s also accompanied by RPM fluctuations of 50-100 RPM. Port engine only, sounds like it’s coming from the air intake. I think it’s been doing it since I purchased (in June), but didn’t think much of it at the time. Any thoughts?