We have had a Drake for about 2 years. Absolutely love it. For a couple of months now, there has been some air gurgling. It started after a particularly cold night (still in winter) and was only in the toilet. It happens near the end of the '3 second flush' when it is pretty much done with the flush and wanting to start the fill. I'm not sure exactly if it is at the end of the flush or start of the fill, but right in that time frame.
It continued like that for a while. My initial thought was that a pipe from the house to the street connection had frozen/cracked/etc. It continued like this for 2-3 or so weeks, then we started noticing the air gurgle sound in the shower drain. The shower drain only gurgles when there has not been any water run through it for a while. It seems to me that somehow the trap is getting emptied. Shower/run some water into the tub, and it won't gurgle (from the shower drain) for several hours. It always continues to gurgle in the Drake.
Things I have done/checked: The drain access (cleanout? - pipe running down into the basement with the capped Y near the floor). It is old cast iron. Didn't seal well at all. Cleaned it up & put in a new screw 3in plug. It is sealed well now. I also checked the roof vent pipe. There were no bird nests blocking the vent.
It is a 90 year old house. This is the only bathroom/toilet. It appears to still flush fine. I am only concerned because it didn't do it at all before, and then suddenly it did. The air gurgle in the shower drain is a bit annoying.