I can almost guarantee you RTK hit it right on the head. It sounds like it is richening up a tad. Not a burble quite as you said. There is a simply fix. I know Gene at Troy Built Models sells a little port cover. It is just a set of spacers to get a thin piece of carbon over the atmospheric vent. All it does is block swirling air from hitting the port. The port is not blocked, but swirling air can not hit it and "confuse" it.

I can promise you if you get the needles perfect on the ground and it still does it in the air, that is it. This is all provided it is NOT a needle issue. Try tweaking the needles like DA says.

Let us know how it comes out,

Norman Ross Jr.