assuming your horn is not leaking and your mpc and reed combo is a good one you might be playing with too relaxed of a embouchure because I was able to gurgle on low b on my alto simply by relaxing my lower lip way too much. also if you have to have your mouthpiece pulled out too far to play in tune that will cause gurgling.If you are playing a real old saxophone some of the newer mouthpieces have too small of a chamber and bore which would make you play sharp and out of tune and then you have to pull out too far which will cause the problem as well. If the neck is loose in the body socket it will make low notes hard to play as well.In order to get the gurgle I had to feel like I was lifting my lower lip away from the reed as I was playing instead of suporting the reed. It is hard for me to do as I am not in the habit of doing this. if I play a fibracell reed my middle and low d will gurgle if I play them soft. I don’t have this problem with other reeds.